Its a ratio of your main keywords against the total words being used in the article. SEO written articles focus on few keywords and keep these in around 3 to 7 percent in article. This give a boast in SE for those keywords.
The percentage of occurrence of a particular keyword on the webpage is calculated as keyword density. It’s an effective SEO tool which is used to gain traffic.
all are right answer but i will define it as, a keyword density is the percentage of the time a keyword or a key phrase appear in the whole content as compare to the total existing word in that content.
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. In the context of search engine optimization keyword density can be used as a factor in determining whether a web page is relevant to a specified keyword or keyword phrase.