A callback, in terms of comedy, is a joke which refers to one previously told in the set. The second joke is often presented in a different context than the one which was used in the initial joke. Callbacks are usually used at or near the end of a set, as the aim is to create the biggest laugh at the end of a comic set. The main principle behind the callback is to make the audience feel a sense of familiarity with the subject matter, as well as with the comedian. It helps to create audience rapport. When the second joke is told, it induces a feeling similar to that of being told a personal or in-joke.
In television, the term callback has come to mean a joke or line that refers to a previous episode (or sometimes, in rare cases, movies). Particularly in earlier sitcoms - though even until the early 1990s — callbacks were rare and often frowned upon by networks, because they threaten to alienate a viewer who is new to the series, or who missed episodes. Seinfeld was one of the first sitcoms to regularly use callbacks in its scripts, although on a level which would often be missed or disregarded by viewers. More recently 30 Rock has employed callbacks to reference fictitious movies and television programs created within the show. Arrested Development became well known by fans for its regular use of callbacks throughout all of its episodes. Of course, the line between a callback and simple continuity can be ambiguous.