Placing excessive keywords on a page copy and the HTML in such a way that it hinders readability and usability of a given page for the sole purpose of boosting the page's rankings in the search engines is termed as keyword stuffing.
Search engines can easily discover that you tried to manipulate your rankings by inserting keywords unnecessarily... This may result a drop in search engine rankings. Very recently, in fact, Google rolled out an update to penalize ‘oper-optimized sites.
“We are also looking for those who abuse it, like too many keywords on a page, or exchange way too many links or go well beyond what you normally expect.” - Googler Matt Cutts