The Collaborative International Dictionary of English v.0.48:

halfback \halfback\ n. (Football)
   A person who plays the position of halfback[2] on a football
   team.

   Syn: running back.
        [WordNet 1.5]

   2. The position of either of two players on a football team
      who typically begins each play behind the line and on
      either side of the quarterback.
      [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

halfback
    n 1: (football) the running back who plays the offensive
         halfback position
    2: (American football) the position of a back on a football team