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

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.

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