What is keyword Density?

Keyword density is defined as the percentage of times a keyword appears on a web page compared to the total number of words on the page.

Keyword density is the measurement in percentage from the number of times a keyword or phrase appears compared to the total number of words in a webpage.

I don't think you can put a general % on keyword density. So long as it reads well and doesn't appeared to be stuffed, it should be fine. Mention it as many times as you can without it appearing forced. There's no doubt that having a keyword appear more times on the page will help Google deduce what the page is about, but similarly anything that would compromise a user experience or attempts to over-optimise for the algorithm can easily be penalised. Saying what number this is though is highly dependent on context, so you can't put a broad figure on an "optimal level" :)