The Collaborative International Dictionary of English v.0.48:

punch line \punch" line`\, n.
   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. [WordNet sense 1]

   Syn: laugh line, gag line, tag line.
        [PJC]

   2. Hence: The final sentence, phrase, or word in any
      narration, speech, or advertisement that produces the
      speaker's intended effect.
      [PJC]

WordNet (r) 3.0 (2006):

punch line
    n 1: the point of a joke or humorous story [syn: punch line,
         laugh line, gag line, tag line]