The period of game play in a football game from the point at which the ball is snapped to the moment the play is called dead.
Scrimmage encompasses all plays that take place in an offensive drive with the exception of yards gained through penalties. Because penalties, when enforced, cancel and replace the outcome of any given play, the play itself is not counted as scrimmage. Scrimmage is most important in determining the statistic of yards from scrimmage, which includes a player's yards gained by way of rushing and passing receptions. An easy way to remember that scrimmage is exclusive to offensive drives is to reference the line of scrimmage, the line on the field from which the offense snaps the ball at the beginning of a play.
Another use of the term scrimmage refers to a practice game, which can involve one or more teams, but is often part of a single team's practice, in which the team's offense runs a series of plays against its own defense.