Extended definition
The percentage occupied by keywords out of total words. The concept is outdated — Google uses semantic context, not density. But too high density = keyword stuffing = penalty.
Context and application
Concept inherited from SEO 2005-2010 when Google literally counted occurrences. Now, BERT and MUM (semantic understanding models) interpret complete context. “SEO” and “search engine optimization” are treated equivalently. Natural use of a range of related terms (synonyms, hypernyms, LSI expressions) is more powerful than exact-match repetition. If you’re tempted to measure density, stop and read the text aloud — if it sounds forced, it’s too much.