icebreaker

Short definition from Wiktionary

Noun

1. 1
   A ship designed to break through ice so that it, or other ships coming behind, can navigate on frozen seas.
2. 2
   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.
3. 3
   A lively song and dance routine at the start of a musical.