App Store Indexation
Indexation is whether and how a keyword is registered as relevant to your app, determining which searches you can possibly appear in.
Indexation is the process by which an app store associates keywords with your app, making it eligible to appear for those searches. If a term isn’t indexed to your app, you cannot rank for it at all — indexation is the precondition for ranking.
On the App Store, terms are indexed from your title, subtitle, keyword field, and developer/in-app-purchase names. On Google Play, they come from your visible title and descriptions. Both stores also auto-generate phrase combinations from your individual words.
A practical ASO check is to confirm your priority keywords are actually indexed (your app shows up somewhere for them) before trying to improve their position.