"In joke" refers to a joke or reference that is understood only by a particular group of people. Other synonyms for this term can vary, depending on the context of the joke. For example, the term "inside joke" is often used interchangeably with "in joke". Additionally, phrases such as "inside reference", "private joke", and "shared joke" can also be used to refer to an "in joke". These terms are often used when a joke is made among a close group of friends or colleagues and is based on personal experiences that only they would understand. The use of "in jokes" can create a sense of unity and exclusivity among group members.