An in joke is a joke whose humour is clear only to those people who are in a group that has some prior knowledge (not known by the whole population) that makes the joke humorous. Also called an inside joke.

This group of people could be, for example:

• a nuclear family or (parts of) an extended family
• just a few people who work together
• people of the same vocation or profession
• residents of a particular town or region
• viewers of a particular television series or cult movie
• readers of a particular book or series of books
• users of the same computer or computer software
• a group of friends
• practitioners of a particular craft, art, or science
• people who read the same encyclopedia

In jokes sometimes appear in film and television. Such jokes may be visual (for example, a movie theatre marquee shown in the background of a scene might display the title of one of the film director's other works), or delivered in dialogue. In-jokes can also take the form of homages to other films or television series.

It is often considered impolite or otherwise meaningless to share in jokes among those who are not "in" on the joke, since any hope of a laughing response will depend on a careful explanation of the joke's circumstances, thereby diluting the joke's effect.

Examples

• Typos introduced by the typo fairy: Professional editors and writers
• Story ideas coming from a mail-order business in Schenectady, New York: Science fiction authors
• The Wilhelm scream, an in joke among movie sound technicians
• Letting out the Magic smoke: among Electrical Engineers