Definition from Wiktionary, the free dictionary
goalpost (plural goalposts)
- (sports) In American football, the tall Y-shaped upright, now usually of fiberglass, at either end of the playing field, through which a football must go in order to score a field goal. Originally, they were H-shaped, with two wooden posts on either side.
- (sports) One of the two side poles of the goal.
Related terms 
vertical part of football goal