Keyword density is a measure of how often you use a keyword in relation to the total text content on the page.  It is measured in a percentage of the total… 3% would mean you use the keyword 3 times in every 100 words.

• If you are over 10% keyword density, you are going to get flogged by the search engine riot police.
• 3% Density for Google, 5% for Yahoo or MSN are typically a good ratio.Remember, keyword density is important (even to your Adwords quality score), but it is just one snowflake in the SEO snowstorm.
• Remember, Google uses slaves kidnapped by alien spacecraft to crawl the internet. If it doesn’t sounds good (i.e. make sense) there is a good chance these Borg-type indexers will notice and penalize your site. All Praise Google!

See Also: Keyword Stuffing