An in-joke, also known as an inside joke or in joke, is a joke whose humour is clear only to people who are in a particular social group, occupation, or other community of common understanding. It is an esoteric joke which is humorous only to those who know the situation behind it.
In-jokes may exist within a small social clique, such as a group of friends, or extend to an entire profession such as the film or professional wrestling industries, or a particular sporting endeavour. Even an ethnic or religious group may have its own in-jokes.