Keyword density refers to the percentage of times a specific keyword or phrase appears in a piece of content, relative to the total number of words. It is a measure of how often a keyword is used and can impact the search engine ranking of a webpage. A high keyword density can indicate that a webpage is relevant to a particular topic, but excessive use of keywords can also be seen as spammy and may result in a lower ranking. Striking a balance between using keywords effectively and maintaining a natural flow of language is key to optimizing keyword density for SEO purposes.