The excessive, unnatural use of keywords on a web page for search engine optimization purposes.
Search engines rank web pages based on their relevance to a given keyword query. In the early days of search engine technology, relevance was heavily influenced by the number of instances that the given keyword appeared on a web page. Thus, many content creators bombard a page’s body with the same keyword in an attempt to increase search rankings. When search engines gave more credit to easily hidden areas such as ALT text and META tags, these areas were prime targets of stuffers.
However, many SEO practitioners today consider this practice outdated. Search engines may now penalize or ignore such keyword-stuffed pages, save for extremely obscure or lengthy keyword phrases. Search engines algorithms have also evolved to balance on-page criteria (i.e. keywords) against off-page criteria (i.e. inbound links) to favor more natural and credible content.
see also: SEO Tools