What is Keyword Stuffing?

Keyword stuffing refers to the practice of adding superfluous keywords to a web page. The words are added for the 'benefit' of search engines and not human visitors. The words may or may not be visible to human visitors. While not necessarily a violation of search engine Terms of Service, at least when the words are visible to humans, it detracts from the impact of a page (it looks like spam). It is also possible that search engines may discount the importance of large blocks of text that do not conform to grammatical structures (i.e. lists of disconnected keywords).