What it is

Contain is a defensive job: a player, usually the edge defender, stays outside and prevents the ball carrier or quarterback from running around the end.

In a game

The defender must not go too deep into the backfield. He holds his position and forces the runner back toward the other defenders, who come from the inside.

More detail

Against a mobile quarterback, a contain rusher keeps the pocket closed. If he fails, the runner turns the corner and gains yards.

Animated diagram

1. The quarterback rolls out to his right, trying to get around the edge.
2. The edge defender does not rush deep. He stays outside and keeps the quarterback inside.
3. The runner has nowhere to go, because the other defenders close in from the inside.