A puck that has left the rink or that has been caught by a player.
According to NHL rules, puck is considered out of bounds when it leaves the playing area or strikes any obstacles above the playing surface other than the boards or glass. The puck will be called dead by officials and play will resume by way of a face-off. The face-off takes place at the nearest face-off spot adjacent to where the puck left play, with the exception that if the puck left the rink off the stick of an attacking player, the face-off will occur at the face-off post closest to the attacking player's zone.
Though strictly referring to a puck that has gone out of bounds or that has been caught in the hand of a player, a puck is typically "dead" during any stoppage of play from the moment a referee's whistle blows. This may or may not result in a penalty. Most dead pucks either result in a face-off or end with possession of the puck by a goalie to put into play.