App store optimization has its own vocabulary. Understanding these ASO terms is the difference between making data-driven decisions and guessing. This glossary covers every metric, concept, and feature you will encounter when optimizing an iOS app — whether you are doing ASO for the first time or deepening your existing knowledge.
Terms are organized into eight categories. Each definition is written in plain English, with context on why the term matters for your ranking and conversion rate. Use the table of contents to jump directly to any section.
Core ASO Metrics
The numbers you track to measure how well your app is performing in the App Store.
Impressions
The number of times your app listing is displayed to a user in App Store search results, the Today Tab, category browse pages, or top charts — without necessarily being tapped. Impressions measure raw visibility and represent the top of your ASO funnel. Every impression is an opportunity to earn a tap.
Tap-Through Rate (TTR)
The percentage of users who tap your app listing after seeing it in search results, calculated as (Taps ÷ Impressions) × 100. In Apple Search Ads, TTR is the primary signal of creative and keyword relevance. A high TTR indicates that your icon, name, and subtitle are compelling for that specific search term. Industry benchmarks typically range from 2–8% depending on category.
Related: Impressions, Conversion Rate, App Icon
Conversion Rate (CR / CVR)
The percentage of users who download your app after viewing your full product page, calculated as (Installs ÷ Product Page Views) × 100. This is the most important on-page ASO metric — it measures how well your screenshots, description, ratings, and video persuade users to install. The App Store algorithm uses conversion rate as a quality signal; a higher CR directly improves keyword rankings.
Related: TTR, Product Page Optimization, Screenshots
Download Velocity
The rate at which an app accumulates new downloads within a specific time window — measured in installs per day or per week. The App Store algorithm treats high download velocity as a strong relevance signal, rewarding it with improved keyword rankings and category chart positions. A sudden spike from press coverage or App Store featuring can produce lasting ranking gains that persist well after the spike subsides.
Organic Installs
Downloads that occur without paid advertising — sourced from keyword search, category browsing, top charts, similar-app recommendations, or the Today Tab. Organic installs are the primary output of ASO work. A well-optimized app typically generates 60–80% of its installs organically. The ratio of organic to paid installs is a key health metric for long-term sustainability.
Organic Uplift
The secondary increase in organic downloads that occurs as a direct result of a paid UA campaign. When paid installs boost download velocity and keyword rankings, the improved visibility generates additional organic installs beyond what the campaign itself produced. Organic uplift is the multiplier effect that justifies UA spend beyond its direct ROI and explains why strong ASO makes paid campaigns more efficient.
Visibility Score
A composite metric (0–100) calculated by ASO tools that aggregates an app's keyword rankings across an entire keyword set, weighted by search volume. It produces a single number representing overall organic search presence. Tracking visibility score over time is one of the clearest ways to measure the aggregate impact of metadata updates and new keyword targeting.
Keyword Research Terms
The vocabulary of finding, evaluating, and prioritizing keywords before writing your metadata.
Search Volume / Apple Search Popularity (ASP) Score
A measure of how often users search for a specific term in the App Store within a given country. Apple exposes this natively as the Apple Search Popularity (ASP) score — an exponential scale from 5 to 100 surfaced in Search Ads keyword suggestions. Because the scale is logarithmic, the difference between an ASP of 60 and 70 represents far more search traffic than the difference between 10 and 20. Third-party tools estimate absolute monthly search volumes using this signal.
Source: Apple Search Ads
Keyword Difficulty
A 0–100 score indicating how competitive it is to rank in the top 10 results for a given keyword. Difficulty is calculated based on the download history, ratings, and keyword saturation of apps already ranking for that term. New or small apps should prioritize keywords with difficulty scores in the 20–50 range while building their download history and authority.
Chance Score
An ASO tool metric (0–100) estimating a specific app's probability of ranking in the top 10 for a given keyword, combining the app's own strength with the keyword's difficulty. A high chance score on a high-volume keyword represents an immediately actionable growth opportunity. Use chance scores alongside difficulty to build a keyword shortlist prioritized by impact.
Long-Tail Keywords
Multi-word keyword phrases (typically 3+ words) that describe a specific user intent — for example, “habit tracker for students” vs. “habit tracker.” Long-tail keywords have lower search volume but typically lower competition and higher conversion rates because they capture more specific intent. They are critical for new apps building early ranking history before they can compete for high-volume head terms.
Keyword Relevance
How topically aligned a keyword is with an app's content, category, and user base, as inferred by the App Store algorithm. High relevance means the algorithm will readily index your app for that term. Targeting irrelevant high-volume keywords violates Apple's developer guidelines and typically produces poor conversion rates — users who search irrelevant terms rarely convert into retained users.
Indexing
The process by which the App Store algorithm reads and catalogues an app's metadata, associating it with specific keyword queries. An app is “indexed” for a keyword when it appears in search results for that term — even on page 10. The goal of metadata optimization is to get indexed for relevant terms and rank highly enough to receive meaningful impressions. New keyword field updates take days to a few weeks to fully index.
KEI (Keyword Efficiency Index)
A composite metric that identifies the best keywords to target by combining high search volume with a high probability of ranking. KEI surfaces keywords at the intersection of opportunity and achievability — the practical sweet spot for keyword selection. Use it to cut through large keyword lists and identify the 20–30 terms most worth including in your metadata. Kōmori's keyword tools can help you generate and evaluate keyword combinations efficiently.
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Metadata & On-Metadata Optimization
The fields and assets you control directly in App Store Connect — the primary levers of ASO work.
On-Metadata Factors
All ranking and conversion factors that are directly edited by the developer through App Store Connect — including the app name, subtitle, keyword field, description, icon, screenshots, and preview video. These are the primary levers of ASO work and the ones you have full control over. Contrast with off-metadata factors, which are driven by user behavior.
App Name / Title
The primary text field for an iOS app, limited to 30 characters. The app name carries the highest keyword ranking weight of any metadata field — keywords here receive the strongest indexing signal. It is also the first text users see in search results, making it critical for both discoverability and conversion. Best practice: lead with the brand name, follow with a 1–2 keyword descriptor. Use Kōmori's metadata counter to stay within character limits before submitting.
Subtitle
An iOS-specific metadata field of up to 30 characters, displayed directly below the app name in search results and on the product page. The subtitle is the second most powerful keyword field in the App Store — keywords here carry more ranking weight than the keyword field or description. Because it is visible in search results before a user taps, it serves a dual purpose: improving indexing and improving click-through.
Keyword Field
A hidden 100-character iOS metadata field visible only to the App Store algorithm, not to users. Keywords entered here (comma-separated, no spaces after commas) tell the algorithm additional terms the app is relevant for. The field should not repeat words already used in the name or subtitle — the algorithm does not compound repetition, so every character should introduce new search vocabulary.
Promotional Text
An iOS metadata field of up to 170 characters that appears above the description on the product page. Unlike other text fields, it can be updated at any time without submitting a new app version — making it ideal for time-sensitive announcements. It is not indexed for keyword ranking and exists purely as a conversion tool to communicate offers, new features, or social proof.
App Description (Long Description)
On iOS, the full description (up to 4,000 characters) is displayed on the product page but is not indexed for keyword ranking by Apple's algorithm. Its role is purely conversion — persuading users to download. On Google Play, the long description is indexed and functions as a keyword optimization field. This is one of the most important platform differences to internalize.
Screenshots
Up to 10 images displayed on the product page. The first 2–3 screenshots are visible in search results without tapping, making them a primary driver of conversion rate. As of 2025–2026, Apple's algorithm actively reads text inside screenshots using OCR — adding a keyword ranking function to what was previously a purely visual asset. Apps that added keyword captions to screenshots reported up to a 22% boost in search visibility within 30 days.
App Localization
The process of adapting all metadata — name, subtitle, keyword field, description, screenshots — for a specific country and language. Apple treats each locale as an independent keyword index, so properly localized apps can rank for entirely different keyword sets in each market. Localization is consistently one of the highest-ROI ASO activities. Even localizing metadata alone (without translating the app UI) unlocks significant download volume in major non-English markets.
Off-Metadata Factors
Ranking signals driven by user behavior — you can't edit them directly, but you can influence them through product quality and strategy.
Off-Metadata Factors
Ranking and visibility signals that exist outside the metadata the developer directly controls. Includes download velocity, ratings and review scores, retention rates, uninstall rates, and engagement metrics. These factors cannot be directly edited but can be influenced through product quality, marketing strategy, and how and when you communicate with users.
Uninstall Rate
The percentage of users who delete an app within a defined period after installing it. A high uninstall rate signals to the App Store algorithm that the app is not delivering on its promise — negatively impacting keyword rankings and featuring eligibility. Misleading screenshots or descriptions that overpromise the app's capabilities are a common driver of elevated uninstall rates.
Retention Rate
The percentage of users who continue using an app after a defined period post-install — typically measured at Day 1, Day 7, and Day 30. App stores treat retention as a proxy for app quality. Strong D7 and D30 retention correlates with improved algorithmic ranking preference. Low retention negatively impacts organic rankings even when raw download numbers are strong.
New App Boost
A temporary algorithmic advantage Apple grants to newly launched apps, improving their keyword ranking positions for a short window after launch — typically days to a few weeks. The boost helps new apps gain initial visibility before they have established download history. Maximizing metadata quality and conversion optimization before launch is critical to capitalizing on this window, as it cannot be recovered once it expires.
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App Store Connect & Apple-Specific Terms
Features and platforms that are unique to Apple's ecosystem.
App Store Connect
Apple's web-based developer portal for submitting, managing, and analyzing iOS apps. It is the sole authoritative source for all app metadata, pricing, availability, in-app purchases, TestFlight builds, and App Store analytics. All on-metadata ASO changes — name, subtitle, keyword field, screenshots — are made exclusively through App Store Connect.
App Store Connect Analytics
The native analytics dashboard within App Store Connect providing Apple's first-party data on impressions, product page views, downloads, in-app purchases, and retention. Data includes traffic source breakdowns across App Store Search, Browse, Referral, and Apple Search Ads. It is the authoritative ground truth for App Store performance data — and the primary source for calculating your organic conversion rate.
Primary & Secondary Category
The categories under which an app is classified in the App Store. The primary category determines where an app appears in category-specific top charts and browse pages. An optional secondary category enables the app to appear in two separate browse contexts and two separate top chart lists. Some developers strategically choose a less competitive primary category to achieve higher chart positions with the same download volume.
In-App Events
Time-limited activities within an app — competitions, live events, seasonal content, or major feature launches — published on the App Store product page and appearing in search results and the Today Tab. Apple's algorithm indexes in-app event titles and descriptions for keyword ranking. They also display as expanded cards for returning users in search results, effectively increasing listing real estate without a full metadata update.
Apple Search Ads (ASA)
Apple's native paid advertising platform for the App Store, operating on a cost-per-tap (CPT) model. Ads appear in four placements: Search Results, Search Tab, Today Tab, and Product Page. Strong organic ASO reduces CPT bids because Apple's algorithm rewards relevance between ad keywords and app metadata — making ASO and paid UA directly complementary, not competing activities.
Today Tab & App Store Featuring
The Today Tab is the App Store homepage, curated entirely by Apple's editorial team. Featuring — hand-selecting exceptional apps for the Today Tab, category collections, and “Apps We Love” sections — is not paid and cannot be directly purchased. Featured placement dramatically increases download velocity and can produce lasting keyword ranking gains. Apps with exceptional design, strong ratings, and active in-app events are more likely to receive consideration.
Ratings & Reviews
The most visible trust signals in the App Store — and a direct input to both ranking and conversion.
Average Rating / Star Rating
The weighted average of all user ratings (1–5 stars), displayed prominently in search results and on the product page. One of the most visible trust signals in the App Store. Apps below 4.0 stars see significant conversion rate depression. Apple's algorithm also uses rating score as a ranking factor — making star rating both an off-metadata ranking signal and a direct conversion lever.
Rating Count
The total number of user ratings an app has received. Rating count signals credibility and social proof. An app with 4.5 stars and 10,000+ ratings is treated far more authoritatively by both the algorithm and potential users than one with 4.5 stars and 30 ratings. Growing your rating count is as important as improving your average star rating.
iOS Rating Prompt (SKStoreReviewRequest)
Apple's native in-app API that displays a standardized rating dialog to users. Apple limits this to a maximum of 3 prompts per 365-day period per app. The timing is fully developer-controlled. Triggering it at high-satisfaction moments — after a completed goal, after a successful purchase, after a session milestone — dramatically improves the average rating received compared to prompting during onboarding.
Review Response
The developer's public reply to a user review, visible to all potential downloaders on the product page. Responding to reviews — especially negative ones — signals that the developer is engaged and maintaining quality. The conversion impact on users reading the product page is well-documented: a thoughtful response to a 1-star review reassures hesitant buyers more effectively than additional screenshots.
A/B Testing & Experimentation
Apple's built-in tools for testing product page variants and measuring conversion improvements.
Product Page Optimization (PPO)
Apple's native A/B testing tool in App Store Connect, allowing developers to test up to 3 treatment variants of their default product page against the original — using a developer-defined percentage of organic traffic. PPO can test icon, screenshots, and preview video, but not the app name, subtitle, or keyword field. Results include statistical confidence levels calculated by Apple.
Custom Product Pages (CPP)
Up to 35 alternative versions of an app's App Store product page, each with unique screenshots, preview video, and promotional text — while sharing the same name, subtitle, and keyword field. Originally designed for paid traffic deep-links, CPPs in 2025–2026 can appear in organic search results when assigned target keywords in App Store Connect. Each CPP tracks its own impressions, views, and conversion rate independently.
Statistical Significance
The probability threshold at which a measured difference in conversion rates between PPO variants can be attributed to the changes made rather than random variation. Apple presents confidence levels in App Store Connect PPO reporting. Publishing a winning variant before reaching sufficient significance — typically 95%+ — risks implementing a change based on noise rather than real user preference.
Technical & Distribution Terms
The infrastructure and frameworks behind iOS app distribution, attribution, and measurement.
MMP (Mobile Measurement Partner)
A third-party attribution platform (AppsFlyer, Adjust, Branch) that tracks the full user acquisition journey — linking an install to the specific ad, campaign, creative, and keyword that drove it. MMPs are essential for calculating true ROI on paid UA and for separating paid installs from organic installs. On iOS, MMP attribution uses Apple's SKAdNetwork framework.
SKAdNetwork (SKAN)
Apple's privacy-preserving attribution framework, introduced as the primary measurement solution following iOS 14's App Tracking Transparency changes. SKAdNetwork sends install conversion signals from Apple's servers directly to ad networks without exposing individual user data. Developers must configure a conversion value schema to extract useful campaign-level performance data from the postbacks it generates.
App Tracking Transparency (ATT)
Apple's iOS 14.5+ framework requiring apps to display a standardized permission dialog before tracking users across apps and websites for advertising purposes. Users who deny consent cannot be attributed via traditional IDFA-based methods. ATT opt-in rates typically range from 25–40%, directly affecting the measurability and efficiency of paid UA campaigns. See Apple's ATT documentation for implementation requirements.
TestFlight
Apple's official beta testing platform, integrated with App Store Connect, allowing developers to distribute pre-release builds to up to 10,000 external testers and 100 internal testers. From an ASO perspective, TestFlight is used to validate new features before release and gather qualitative feedback that informs metadata and creative decisions. TestFlight installs do not count toward App Store ranking metrics.
How These ASO Terms Connect: The Full Funnel
Every term in this glossary sits somewhere in the ASO funnel. Understanding where each metric lives — and how it feeds into the next — is what separates developers who optimize systematically from those who guess.
Keyword Ranking
↓
Impressions → TTR → Product Page Views → Conversion Rate → Installs
↓
Retention & Engagement
↓
Ratings & Reviews
↓
(feeds back into ranking)On-metadata factors (name, subtitle, keyword field) drive keyword ranking and impressions. Visual assets (icon, screenshots, video) drive TTR and conversion rate. Product quality drives retention and ratings. Ratings feed back into ranking. Every metric is connected — optimizing one in isolation without understanding the full loop produces diminishing returns.
For a practical walkthrough of how to apply these concepts, read our complete ASO guide for iOS developers. For a comparison of the tools available to manage this process, see our review of the best ASO tools.