In-joke

An in-joke, also known as an inside joke or a private joke, is a joke with humour that 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, therefore, an esoteric joke, only humorous to those who are aware of the circumstances behind it.

Typically, inside jokes use a reference in the punchline to imply that which is associated with the reference. Often, this reference refers to the punchline of another joke which was already heard by the ingroup.

In-jokes may exist within a small social clique, such as a group of friends, or extend to an entire profession or other relatively large group. When the ingroup only includes people which heard the previous portion of a comedic set, the type of inside joke is known as a callback.

An example is:

Q: What's yellow and equivalent to the axiom of choice?

A: Zorn's lemon.

Individuals not familiar with the mathematical result Zorn's lemma are unlikely to understand the joke. The joke is a pun on the name of this result.

Ethnic or religious groups may also have in-jokes.