Definition: Placing excessive amounts of keywords into the page copy and the HTML in such a way that it detracts from the readability and usability of a given page for the purpose of boosting the page’s rankings in the search engines. This includes hiding keywords on the page by making the text the same color as the background, hiding keywords in comment tags, overfilling alt tags with long strings of keywords, etc. Keyword stuffing is just another shady way of gaming the search engines and, as such, its use should be strongly discouraged.
FirstClickSEO Commentary: A ‘black hat’ technique that will get you banned. This happened to BMW, check out our history page to learn more.