Black hat SEO refers to a set of practices that are used to increases a site or page's rank in search engines through means that violate the search engines' terms of service. The term "black hat" originated in Western movies to distinguish the "bad guys" from the "good guys," who wore white hats (see white hat SEO). Recently, it's used more commonly to describe computer hackers, virus creators, and those who perform unethical actions with computers.