I have heard quite a few people ask about keyword density. It spurs from either not exactly understanding the term or how they can apply the concept to their site. However, what does that mean for you and what you want to write about?
According to Wikipedia:
Keyword density is the percentage of times a keyword or phrase appears on a web page compared to the total number of words on the page.
When you write an article, you have a topic. Usually that is what your keywords pertain to, but some people write articles and put a relevant keyword. For example, if I was writing about designing a site and decided instead of using web design as a keyword and linking it to another site, I want to do that to web hosting. I could and that would be a keyword.
It does not have to do with tagging your posts. Tagging is something like categories, they are both used for organizing your site – categories for general topics, and tagging is more specific topics.
For anyone who has professionally written for article companies that focus on keyword density, a lot of them ask that a keyword not be used more than 1.6%, but at the least .6%. This could mean you could use that keyword any where from 1 to 5 times depending on how short or long your article is. After that, search engines like Google may believe you are trying to keyword stuff your article which is a big frown face no-no.
The beauty of language is that there are many words that are similar and you can avoid keyword over usage by consulting a Thesaurus.
If you are concerning about keyword density, you can use the keyword density analyzer, which is free for anyone to try out.
If you are using WordPress, you might want to try out Keyword Statistics.
What other tools might make monitoring your sites keyword density more convenient?