The Collaborative International Dictionary of English v.0.48:

goalpost \goalpost\, goal post \goal post\, n. (Sport)
   One of two posts supporting a crossbar which forms a goal[3],
   especially in American football; also, in football the entire
   structure consisting of the posts, crossbar, and two
   uprights. To score a goal by kicking the football, the ball
   must pass above the crossbar and between the vertical lines
   formed by the uprights.
   [PJC]

WordNet (r) 3.0 (2006):

goalpost
    n 1: one of a pair of posts (usually joined by a crossbar) that
         are set up as a goal at each end of a playing field