Keyword Density is one of the important On Page SEO factors.It is a value measured in percentage i.e the number of times a keyword displays on a page to the total number of words that appear in a page.For SEO, keyword density is used in order to determine if a web page is relevant to a particular keyword.
However though there are other factors which search engines concentrate on to determine relevancy,keyword density also needs to be given its due importance while performing On Page SEO.

Often,there is a standard question when it comes to keyword density.What is the ideal keyword density
for a website?This question has been put forward in the following forum http://forums.seroundtable.com/showthread.php?t=1100.Number of replies have been posted in this forum.Following is one of the informative replies by a member of that forum.

"Ok heres a tip, say if i was on page 2 on Google at number 11 and i wanted to get onto page 1 at the
number 10 spot and i have done everything by the book, title tags, META tags, on-site optimization and i
still couldn't get on the first page how would i do it?Well i would check the keyword density of the
person in the number 10 spot and simply just match with them.If you have more backlinks than them and your
site is better optimized than them you'll knock them of the number 10 spot before you know it.So that is
the easiest way to determine the keyword density, just check your competitors and match it."