A measure of the number of times a keyword is repeated on one webpage. It’s calculated by dividing the number of times a keyword appears on a web page by the total number of words on the page.
Previously, high keyword density was thought to be good for SEO because it would give search engines clarity on the site’s content. However, people were “keyword stuffing,” or repeating a keyword so many times and so unnaturally that it created a poor user experience, so Google stopped using keyword density as a ranking factor. Now, search engines are more likely to consider a website that repeats or stuffs keywords spammy and penalize it in SERPs.