'punch line' definitions:

Definition of 'punch line'

(from WordNet)
noun
The point of a joke or humorous story [syn: punch line, laugh line, gag line, tag line]

Definition of 'punch line'

From: GCIDE
  • 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]