In years past, one of the most common practices of black hat search engine optimizers was that of keyword stuffing.
The practice entailed "stuffing" (or including) as many relevant keywords into a page as was possible in the hope that search engines would consider the individual page more relevant to the user's query. It used to work but keyword stuffing has all but faded out of practice (well, for the most part).
Last month, Bing updated their Webmaster Guidelines addressing the use of keyword stuffing, indicating that it may demote sites or delist sites entirely that use keyword stuffing.
Here's Bing's updated Webmaster Guideline on keyword stuffing:
"When creating content, make sure to create your content for real users and readers, not to entice search engines to rank your content better. Stuffing your content with specific keywords with the sole intent of artificially inflating the probability of ranking for specific search terms is in violation of our guidelines and can lead to demotion or even the delisting of your website from our search results."