Keyword density is a simple metric: the number of times a specific keyword or phrase appears on a page divided by the total number of words, expressed as a percentage. If a 1,000-word article uses the phrase 'email deliverability' ten times, its keyword density for that phrase is 1 percent. The concept dates to the early days of SEO when search engines relied heavily on raw term frequency.
Modern search engines are far more sophisticated than a keyword counter. They use natural language understanding, synonyms, entities, and context to judge relevance, so there is no magic density percentage that guarantees rankings. Chasing a specific number is a dated tactic. Still, density remains a useful diagnostic for spotting two opposite problems: keyword stuffing and under-optimization.
Keyword stuffing - cramming a term unnaturally often - reads badly to humans and can trigger spam signals that hurt rankings. On the other end, a page that never clearly states its target topic may fail to signal relevance at all. A keyword density check helps you see whether your primary term appears naturally enough to be clear, without being repeated to the point of awkwardness.
The practical approach is to write for people first, cover the topic thoroughly, and use the target keyword plus natural variations and related terms where they fit. Then use density analysis as a sanity check rather than a target. Comparing your density against top-ranking pages can reveal whether you are wildly over- or under-using a term relative to what already succeeds.