fullback

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Football An offensive backfield player whose position is behind the quarterback and halfbacks and who primarily performs offensive blocking and line plunges.
• noun Sports A primarily defensive backfield player in field hockey, soccer, or Rugby whose position is near the defensive goal or goal line.

from The Century Dictionary.

• noun In foot-ball. See back, 12.

from the GNU version of the Collaborative International Dictionary of English.

• noun an offensive football player who plays farther behind the line of scrimmage than the half-back. They are used primarily for blocking and line plunges.
• verb (Football) To play the position of fullback on a football team; to be the fullback.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun American football An offensive back whose primary jobs are to block in advance of the halfback on running plays and for the quarterback on passing plays.
• noun field hockey A defensive player who assists the goalie in preventing the opposing team from scoring.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (American football) the position of a back on a football team
• noun (football) the running back who plays the fullback position on the offensive team
• verb play the fullback