Keywords as a percentage of indexable text words.
Note – Keyword density can vary when analyzing a page for various search engines. Different search engines may use different stop words or stemming techniques, resulting in a word being counted by one engine and not counted by another. Aside from the problem of determining what constitutes a word, there is also the problem of determining which tags various search engines will support. The most basic scenario might have a search engine using only <body> text. More elaborate scenarios might have a search engine using <body> text, ALT tags, META tags, comment tags, and a handful of other tags.
For example, if a keyword is used 5 times in page with 100 indexable text words, the page has a keyword density of 5%. Generally, keyword density should be high enough that a keyword is recognized as a main topic of a page and not an incidental, unimportant word. At the same time, keyword density should not be too high, as some search engines may penalize a page for keyword stuffing.
see also: SEO Tools