Definition of "heads up"

heads up

interjection
1. (idiomatic) Used as an informal warning, caution, or call for attention; pay attention!
  Quotations
  Heads up, Marines! We got trouble.
  2007 September 25, Bungie, Halo 3, v1.0, Microsoft Game Studios, Xbox 360, level/area: The Ark
noun
plural heads ups
1. Alternative spelling of heads-up
adjective
not comparable
1. (poker) Containing only two players.