In-joke

An in-joke (also known as an in joke or inside joke) is a joke whose humor is clear only to those people who are "inside" a social group or occupation. They may be colloquially referred to as "You had to have been there to think it's funny" moments. It is only humorous to those who know the situation behind it. Inside jokes may exist within a small social clique, such as a group of high school friends, or they may extend to an entire profession (e.g., inside jokes in the film industry).