A design pattern which is carefully crafted to accomplish some result but does not have the user’s interests in mind.
Dark patterns use usability principles and psychological motivation in 'evil' ways, pulling the user towards behaviours good for the author's interests but bad for the user's. Dark patterns include designs that scare or intimidate the user; UI copy that misleads or misrepresents actions; 'bad' defaults and interfaces that hide or conceal information that would help users make informed choices.
Not the same as anti-patterns
Whereas anti-patterns are examples of things not to do because they are ineffective or confusing, dark patterns shouldn't be used because they are usually immoral. The difference is sometimes one of intention: a checkout page that uses a small font for delivery costs might be trying to hide the information (a dark pattern), but it could simply be incompetently designed (an anti-pattern).
