also: mousetrap · trap block · trap coverage
On offense, leaving a defender alone on purpose so you can hit him from the side. On defense, leaving a throw open on purpose so you can jump it. Same idea, opposite sides of the ball.
3 meanings, all below. Jump to the one you heard.
A run play in which a defensive lineman is deliberately left unblocked at the snap, then blocked from the side by a lineman pulling from the other side of the formation. The defender's own penetration is what puts him where the block can reach him.
trap play, mousetraptrap coverageThe block itself: the pulling lineman's angle block on that unblocked man. A trap block is distinct from a kickout by intent, since the trapper is attacking a defender who has already crossed the line rather than sealing one who has not.
trap blockkickout block, log block, wham blockA coverage tactic in which a defender sits on a short outside route and drives on the throw while a deeper defender takes his vertical responsibility. The quarterback is shown an open flat that closes as the ball is in the air.
trap coverage, corner trap, cloud trapthe trap playThe offensive senses are taught at every level. The coverage sense mostly appears from varsity up, which is why a coach can spend years hearing only one meaning.