(football) the person who plays quarterback
"The rookie quarterback dropped back to pass just as the defensive line closed in on him."
(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"
An offensive back whose primary job is to pass the ball in a play.
"The rookie quarterback dropped back and threw a perfect spiral to the receiver on the next down."
play the quarterback
"After the starting signal, the coach told him to step back and play the quarterback for the final drive of the game."
To play the position of quarterback.
"After the backup suffered a sprained ankle, the coach decided to let him start and play as the quarterback for the game."