heads up

Short definition from Wiktionary

Interjection

1. 1
   Used as an informal warning, caution, or call for attention; pay attention!

Noun

1. 1
   Alternative spelling of heads-up.
2. 2
   A warning or call to pay attention; an advisory notice; a notice of what is to happen; a holler.
3. 3
   Involving two players. (usually said when there are only two players in the table; or, when all players except two folded)

Verb

1. 1
   third-person singular simple present indicative of head up

Adjective

1. 1
   Containing only two players.
2. 2
   With head directly above the neck, eyes front.
3. 3
   Alert; vigilant