A term used by Sport-literature individuals to describe a sportsman that can be relied upon to deliver and succeed at the crucial, definining and pivotal moments in his teams' , or his own sporting conquests.
It is most commonly used in Footballing terms - a big game player is someone who can deliver goals, or play well and do his role effectively and inspirationally - and not be over-awed, but create fear and awe in the opposition, without getting swayed in by the occasion , be it a crucial fixture against a big team, or a World Cup Final , or Cup Tie and other such engagements. People say that a great player can be defined most by their ability to be big game players - delivering against the biggest teams, when their teams need it the most, and succeeding against all qualities of opposition.
This term can also be called as 'Big Match Man'.
This term although is mainly attached to Football, it can be attached to other sports as well.