The Collaborative International Dictionary of English v.0.48:

icebreaker \icebreaker\ n.
   1. a ship with a reinforced bow and powerful engines designed
      to break up layers of ice on waterways and keep channels
      open for navigation.

   Syn: iceboat.
        [WordNet 1.5]

   2. A remark or action intended to relieve tension or reduce
      formality when initiating conversation or beginning a
      speech; it is often a humorous or light remark.
      [PJC]

WordNet (r) 3.0 (2006):

icebreaker
    n 1: a ship with a reinforced bow to break up ice and keep
         channels open for navigation [syn: icebreaker, iceboat]
    2: a beginning that relaxes a tense or formal atmosphere; "he
       told jokes as an icebreaker"

The Free On-line Dictionary of Computing (30 December 2018):

icebreaker

    A program designed for cracking security on
   a system.

   See also: ICE.

   [Jargon File]

   (2000-03-18)