Wordnet 3.0
NOUN (2)
1. 
 any game designed to exercise the intellect; 
2. 
 deliberate actions of calculated psychological manipulation intended to intimidate or confuse (usually for competitive advantage); 
- Example: "football players try to play mind games with the opposition"- Example: "the jeweler's mind game is to convince lovers that the size of a gemstone reflects the depth of their feelings"
WordNet (r) 3.0 (2006):
mind game
    n 1: any game designed to exercise the intellect
    2: deliberate actions of calculated psychological manipulation
       intended to intimidate or confuse (usually for competitive
       advantage); "football players try to play mind games with the
       opposition"; "the jeweler's mind game is to convince lovers
       that the size of a gemstone reflects the depth of their
       feelings"