A punch line (also punch-line or punchline) concludes a joke; it is intended to make people laugh. It is the third and final part of the typical joke structure. It follows the introductory framing of the joke and the narrative which sets up the punch line. In a broader sense, "punch line" can also refer to the unexpected and funny conclusion of any performance, situation, or story. The origin of the term is unknown.

Source: Wikipedia