What is a Keyword density?

Keyword density is the measure that expresses percentage of indexable text words. It can be calculated as a ratio between the number of how many the keyword appears and the total number of words shown on the certain web page.

For example, if a keyword appears 4 times on the webpage with 100 text words, the keyword density is 4 %. This measure should be high enough, not too much. The higher percentage indicates search engine spam and pages can be penalized for keyword stuffing