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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, i.e., it 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 friends, or extend to an entire profession. An example is:
This joke is so esoteric that most outsiders could not even confidently guess to which group it might be funny, let alone why. In fact, it is a mathematics joke,  a pun on the name of a famous result, Zorn's Lemma.
Ethnic or religious groups may also have their own in-jokes.
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 in something not explicitly spoken. They may even apologize for doing so to a rookie, directly or indirectly stating that what they were laughing at was an in-joke.