D defs.my
Entry 2 senses Webster, 1913

punch line

/pŭnch līn/ · punch· line · IPA /ˈpʌntʃˌlaɪn/
01 n. The final sentence, phrase, or word in a joke or humorous story that is unexpected and produces the humorous effect; the point of a joke.
  1. 1.
    The final sentence, phrase, or word in a joke or humorous story that is unexpected and produces the humorous effect; the point of a joke.
  2. 2.
    The final sentence, phrase, or word in any narration, speech, or advertisement that produces the speaker's intended effect.