Keyword Density

This term refers to the number of times you use your keywords throughout a
page or a site. If you had a 200 word page that you’re trying to optimize for the
word “peanut butter,” but only said this phrase once throughout the whole page,
you’re not letting Googlebot and it’s spider pals know what you’re talking about at
all…
Alternatively, if 50 of those 200 words (25%) are the keywords themselves, then
you’re really going to be spamming the search engines, as Googlebot will see
right through what you’re trying to do and penalize you for it. Even worse, people
reading such copy would think that you’re not even a native speaker of your
language, and leave the page immediately!
The best course of action would be to aim between 0.5% and 5% of the content
on a page. Any more would sound artificial; any less would be too vague. The
trick is to never let it influence your copy’s effectiveness.
This doesn’t take into account the other places you can put your keywords, such
as the name of the page, the title, in HTML comment tags, as names for the
photos it links to, and of course in the Meta Tags in your HTML code. They’re all
wide-open for you to shove