Keyword Density is the term given to "the measure of how many times a keyword or a keyword phrase appears on a page,in comparison with other words that appear on the page". Keyword Density is an important part of search algorithm and pages with a certain keyword density index are rewarded with better rankings. The whole concept of keyword density has undergone a sea-change because of issues concerning keyword stuffing which have resulted in search engines tightening the belts on too many occurances of phrases in a particular page. Here are a few things you need to keep in mind with respect to Keyword Density:

1. Make sure that you mention the keywords in the copy just enough so that its human readable and acceptable.
2. Make sure that the keywords are not mentioned too many times in the copy.
3. Ensure that there is a decent occurrence of keyphrases and keywords in the page, so its not necessary you avoid putting keywords in where necessary just to avoid search engine filters.
4. Most importantly, write content for the users and not for the search engines.