Bottle Episodes

Bottle Episodes take place entirely within one location, usually the main interior set for a series, and consist largely of dialogue and scenes for which no further preparations are needed. In the case of It's Always Sunny in Philadelphia, this will likely be Paddy's Pub.

The phrase originates from a similar term used in the 1960s on the set of Star Trek. Cast and crew members of the show used the phrase "ship-in-a-bottle episodes" for those that took place entirely on board the USS Enterprise.