When the ingroup only includes people which heard the previous portion of a comedic set, the type of inside joke is known as a callback.

An example is: Individuals not familiar with the mathematical result Zorn's lemma are unlikely to understand the joke.

[3] In-jokes are cryptic allusions to shared common ground that act as selective triggers; only those who share that common ground are able to respond appropriately.

[5] An in-joke can also be used as a subtext, where people in the know may find humor in something not explicitly spoken.

They may even apologize for doing so to a rookie, directly or indirectly stating that what they were laughing at was an in-joke.