Keyword stuffing is the practice of inserting a large number of keywords into Web content and meta tags in the attempt to artificially increase a page's ranking in search results and drive more traffic to the site. A keyword is a significant term that is relevant to the content in question.
Keyword stuffing is considered an unethical SEO (search engine optimization) technique at best and an attack technique at worst. The practice is often used to drive traffic to fraudulent or malicious websites.
Keyword stuffing in content is known as spamdexing. Keywords may be hidden in content through a number of methods, such as matching font color to the background, setting font size to zero or putting it behind an image. Words and phrases used for keyword stuffing are typically relevant when visible to the reader but when keywords are hidden they are often just a list of common search phrases, such as sexual terms and celebrity names.
According to Google, keyword stuffing is not only unethical -- it's also ineffective. Meta tags are no longer used to rank websites, so keywords placed in that field are ignored. Increasingly sophisticated algorithms help Google identify not just when terms are irrelevant but also when they are out of context or overused. Furthermore, if hidden text is detected in content, Google may remove a site from its index so that it doesn't appear in search results at all.
Google offers some advice: "Filling pages with keywords results in a negative user experience, and can harm your site's ranking. Focus on creating useful, information-rich content that uses keywords appropriately and in context."