An in-joke, also known as an inside joke or a private joke, is a joke whose humour is understandable only to members of an ingroup, that is, people who are in a particular social group, occupation, or other community of shared interest. It is an esoteric joke that is humorous only to those who are aware of the circumstances behind it.

In-jokes may exist within a small social clique, such as a group of mates, or extend to an entire profession such as the film or professional wrestling industries, or a particular sporting field or chess team. Ethnic or religious groups usually have their own in-jokes.

Philosophy

In-jokes are cryptic allusions to shared common ground that act as triggers; only those who have shared the common ground provide an appropriate response. An in-joke works to build community, sometimes at the expense of outsiders. Part of the power of an in-joke is that its audience knows that there are those who do not understand the joke.

An in-joke can also be used as a subtext, where people in the know may find humour with something not explicitly spoken. They may even apologise for doing so to a rookie, directly or indirectly stating that what they were laughing themselves hoarse at was an in-joke.