Some additional definitions for game theory:

positive sum game:  The payoff to one player added to the payoff of the other player is
positive (so, theoretically, both players can win -- that is, gain from the game). A type of non-zero-sum game.

negative sum game:  The payoff to one player added to the payoff of the other player is
negative (so, theoretically, both players can lose -- that is, lose from the game). A type of non-zero-sum game.

tit-for-tat -- players do to the other what was done to them.  So if one firms sets a high
price, the other follows.  If one player cheats, so does the other.  Usually in sequential games.