Keyword Density measures how often a keyword appears relative to the total words in a text field — relevant to Google Play's indexed description, but not to the App Store description, which isn't indexed.
Density is usually expressed as a percentage: occurrences of a term divided by the field's total word count. It matters only where text is indexed. Google says the text in a store listing helps users discover an app and tells developers to use SEO best practices in the description, while its store-listing guidance warns against "repetition of words" and its metadata policy lists word blocks among common violations. The App Store description isn't indexed for search, and Apple advises against adding unnecessary keywords to it, so density there has no ranking effect; the keyword field does that job.
Neither store publishes a target density, so any percentage is a practitioner rule of thumb, not policy. The useful reading is relative: in our experience, a core term used naturally a few times across a Google Play description helps signal what the app is about, while a term that dominates the text reads as spam and risks the metadata policy, which warns that "excessive length, detail, improper formatting, or repetition can result in a violation." Practitioners check density before publishing, confirm each priority term appears in context, and rewrite any sentence that exists only to repeat a keyword. The Keyword Density Checker flags high-density words.
Example
A running app's Google Play description mentions "running tracker" once and "run" in nearly every sentence. A density check exposes the imbalance, so the team works "running tracker" and "marathon training" into natural sentences and trims the repeated "run", keeping the copy readable while covering more searches.