Keyword density is the percentage of times any given keyword or phrase appears on a single website’s page against the total number of words on that page. In SEO, keyword density is used to help determine a specific keyword or phrase’s relevancy on that page. For search engines, too little use of a targeted keyword or phrase is seen equally as negative as over use (commonly referred to as keyword stuffing).
For example, if your target keyword is ‘luxury real estate’ and that keyword is used only once or twice on an 800-word count page, you may not see any real ranking consideration from search engines for that phrase. Alternatively, if you use that keyword 20 times on an 800-word page, search engines will feel the keyword is purposely being over used for ranking consideration and unnatural – which can result in content penalties that can negatively affect your ranking.