Premise
From IRC Improv Wiki
Premise is an idea or part of idea that can be paired with an unusual thing along with characters, activity, and location in an initiation line.
When a premise is created in the opening, it reappears in a scene as reference.
A premise initiation or scene is different from an organic scene. With a premise line, an improviser is introducing an offer of an idea. In game-based improv, the scene partner or partners work together to turn the premise into a repeatable game by using techniques such as justification.