Keyword Density
Keyword density is a combination of the number of times a keyword or a keyword phrase, in proportion with other words, appears on a Web page. The more times the keyword appears in relation to the total number of on page words, the greater the overall keyword density. The more times that other words appear, the lower the proportion of keywords, resulting in a lighter keyword density.
Most web designers will aim for a keyword density on a page to be from around 1% - 5% maximum. Any more than that could be classified as 'keyword stuffing' and search engines will penalise a page for this.