Heads-up
A hand, or an entire match, played between exactly two players.
Heads-up play strips away most of the multi-way considerations that shape a full-ring hand: there is no wider field to fold out, no need to weigh several opponents' ranges at once, and position alternates every hand rather than staying fixed. It is the format the push/fold endgame is drilled in, since the final stages of a tournament, and most short-stack all-in confrontations generally, ultimately come down to exactly two players.