The best way to understand these definitions is reading first the right definition, then read people definition.
I have found one, hope it help you:
"Black hat SEO is a practice that increases a 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."