noun

• (nautical) A ship designed to break through ice so that it, or other ships coming behind, can navigate on frozen seas.
  "The steel hulls of ice-breakers are much thicker than those of standard vessels."
• (figurative) A game, activity, humorous anecdote, etc., designed to ease tension and encourage interaction among individuals in a situation where they do not know each other well.
  "The new college hallmates were awkward with each other at first, but after a game of charades as an icebreaker, they were laughing like old friends."
• (theater) A lively song and dance routine at the start of a musical.
  "At the moment the curtain went up there had to be a lively ensemble number of principals and chorus singing and dancing, the traditional “icebreaker” that also covered up the noise of late-arriving ticket holders getting to their seats."