Stop Words are filler words like "the" and "to" — and, in ASO usage, broad terms like "app" — that add no value in the App Store keyword field and waste its characters.
The term comes from information retrieval, where search systems traditionally ignore very common words. Apple publishes no stop-word list, but its guidance names the words that waste keyword-field space: filler such as "the" and "to"; terms too broad for the app, like "app" or "game"; category names; plurals of words already included, which Apple treats as duplicates; and special characters such as # or @ unless they're part of the brand. Apple also says not to repeat words from the app name, subtitle or category, and App Store Connect notes that an app is already searchable by its app name and company name.
Every stop word spends characters in a 100-character field that could hold a rankable term, so removing them is the cheapest keyword gain available. A separate group is prohibited rather than merely wasteful: competitor app names, unauthorized trademarks, celebrity names and irrelevant terms, which Apple cites as a common reason for rejection. Practitioners audit the field word by word before each release — remove spaces after commas, drop anything already in the name or subtitle, and reinvest the saved characters. The Keyword Field Optimizer strips filler, duplicates and words already in the title or subtitle.
Example
A recipe app with "Easy Recipes" in its name ships a keyword field reading "the, recipe, app, cooking for beginners". "The", "for" and "app" are filler, "recipe" duplicates "Recipes" in the name, and the spaces after commas are wasted. Cleaned to "cooking,beginners", the field frees room for "vegan,keto,budget".