Any action which delays the next play. On offense, this means failing to snap the ball before the play clock reaches zero. It may also include spiking the ball.
On defense, it occurs when a player hinders the offense in hurrying to make the next snap. This happens most often in the last two minutes of a half when the offense is trying to go down the field in a hurry. The defense can also be flagged for a delay of game if a player spikes the ball after the end of a play.
On special teams, it happens when the return team runs after signaling for a fair catch, or the defense does not unpile in a timely manner after the play ends.