The frequency of a keyword relative to a text’s length. It is a descriptive calculation, not a recommended optimization target.
Keyword density describes how often a chosen word or phrase occurs relative to the length of a text. It is a counting measure. Different tools may count phrases, word forms, navigation and body text differently, so their percentages may not be directly comparable.
The number does not tell you whether a page answers its reader’s question. A necessary product name can recur naturally in a technical explanation, while a much shorter page can repeat a phrase awkwardly. Context, meaning and readability are more useful editorial tests than trying to reach a predetermined percentage.
Google’s spam policies identify unnatural repetition intended to manipulate rankings as keyword stuffing. That does not establish a safe numerical threshold below which all writing is good or above which every page is spam. Do not use a density target to justify adding phrases that make the explanation less useful.
Takeaway: Use a repetition count only as a prompt to reread the copy. Keep terminology where it helps understanding and remove forced repetition rather than chase a supposedly ideal percentage.
Example
An editor sees that a product name appears often in assembly instructions. Some occurrences are necessary to distinguish that product from a compatible accessory. In a separate sales paragraph, repeating the full name in every sentence may add no clarity. The same counting metric cannot decide which use is helpful without reading the text.
What to check
If a tool flags density, inspect what it counted and review the actual passage. Check whether a reader can understand the subject without awkward repetition. Do not replace precise terms with vague synonyms merely to lower a score, or add words merely to increase it.
For more detail, see Spam policies for Google web search.