The Collaborative International Dictionary of English v.0.48:

quarterback \quarterback\ n. (Football)
   The player on a football tream who chooses the offensive play
   and receives the ball from the center at the start of each
   down. Compare halfback and fullback.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

quarterback \quarterback\ v. i. (Football)
   To play the position of quarterback, in a football game.

   Syn: play the quarterback, be the quarterback.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

quarterback
    n 1: (football) the person who plays quarterback [syn:
         quarterback, signal caller, field general]
    2: (American football) the position of the football player in
       the backfield who directs the offensive play of his team;
       "quarterback is the most important position on the team"
       [syn: quarterback, signal caller, field general]
    v 1: play the quarterback