Douglas Harper's Etymology Dictionary
Wiktionary
n. A piece of humour that is understood only by a group of people.
WordNet
n. a joke that is appreciated only by members of some particular group of people
Wikipedia
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.
Usage examples of "in-joke".
He even makes a mathematics in-joke about this being one of the earliest practical applications of binary notation.