If your website content is reasonably well written, it should not be too difficult to manipulate it so that it has the optimum keyword density. After manipulation the content should read well to humans and not just search engine spiders. To understand what we are trying to achieve by manipulating your content you must first understand the term keyword density.

Simple Definition of Keyword Density or Keyword Frequency
Keyword density is a measure of the number of times your keyword appears on a page expressed as a percentage of the total word count of that page. For example, if your page has 100 words, and your keyword phrase appears 5 times, its density is 5%. So when you hear someone say "keyword density", that's normally what they're talking about.

Complex Definition of Keyword Density or Keyword Frequency
The more complex measure of keyword density takes into account the text components in the HTML of the page (i.e. the meta tags: Title, Keywords, Alt Text, Description, and Comments). When using this measure, you don't just count the words your visitor sees; you also count the words in your Meta tags. For example, if you have 100 words on your home page, 10 words in your Title tag, 20 words in your Description tag, 70 words in your Alt tags, and 10 words in your Comments tag, your total word count for the page is 100 + 10 + 20 + 70 + 10 = 210.

Similarly, when counting keywords, you don't just add up the number of times a visitor will see your keyword, you also count the number of times that keyword appears in your Meta tags. For example, if your keyword appears 5 times in the home page copy, 3 times in the Title tag, 5 times in the Description tag, 30 times in your Alt tags, and twice in your Comments tag, your total keyword count is 5 + 3 + 5 + 30 + 2 = 45. So with a total word count of 210 and a keyword count of 45, your keyword density is 45/210 x 100 = 21%. It is argued that this measure of keyword density is more relevant as the search engines measure density in this fashion.