Keyword Density, as a publisher you should know what is Keyword Density exactly. This term's getting pupolar since Google announced their latest google penguin algorithm. Keyword Density means "Percentage of Keywords" found on a content compared to the entire text on the page. - Let's say, i publish a new post on my blog and i put too many keywords on the content (poeple say it's stuffing), maybe i put word "opera" as much 10 words. As such, it may cause something bad for my blog as Google considers my blog is (something like) a spam blog. However, if i put not too many keywords on the content and the keywords are relevant, then Google might consider my post is usefull and good to be shown on the Search result page. - The keywords here are not limited to normal text only, but it could even be an anchor text, and an anchor text is good to tell Google what the content you write is about exactly for it points to another content/page, therefore you better add an anchor text wisely.
Poeple said a good Persentage of Keyword density to each crawler, like Google, Yahoo, Bing, and so on, isn't the same. For example, the ideal Keyword density to Google is about 6%. Seo experts said "Keyword Density should be 4 to 5% in order to google sees it good".
Here i list some usefull online tools that you can take advantage of them to find out if your keyword density is good or otherwise bad.