A referee who lines up about five yards off the line of scrimmage on the defensive side of the ball.
This is probably the busiest referee position on the field. This official has to make sure all the players are dressed in proper gear at all times during play. This referee also has to make sure the offense has the proper number of players on the field during each play. Most holding calls, either on the part of offensive or defensive linemen are the responsiblity of the umpire. The umpire also monitors the condition of the football during gameplay and decides when it is time to change the ball. All official timeouts are recorded by the umpire.
Sometimes referred to as the 12th man because some teams incorporate the extra body into their defensive or offensive plans, although the referee has no official participation in the actual play. The San Francisco 49ers admitted to using this strategy in the 1990's. It is not illegal.