Alibi is a concept in game design theory that relates to the roleplaying contract (the fiction that what happens in game is separate from what happens in real life). "Alibi" denotes the excuse for interaction that enables players to fully engage in a game as characters, and helps bypass the normal social rules.
For example, it might be considered creepy for a player to say, in a game about a pedophile serial murder, "I'd really like to play the pedophile serial murderer. That sounds awesome." However, if the organizer assigns roles, and says, "Hey you! I need you to play the pedophile serial murderer," then the organizer has provided the player with an alibi for portraying a morally reprehensible figure. The organizer action is the excuse -- the alibi -- that the player needs in order to go into darker territory.