The Adversary
The adversary is a fictitious, omniscient structure that is often used to test algorithms. It can provide an algorithm's input, and because it is omniscient it can choose the set of inputs that will make a given algorithm perform its worst. Randomized algorithms have some advantage against such an adversary; because algorithms or responses have different weaknesses, random selection of a response makes it impossible for an adversary to know which is the worst set of inputs, because that set varies depending on which algorithm or response is chosen.