A set of strategies or actions in which each firm makes the best decision for itself that it can, taking the actions of its competitors as given.
http://450.aers.psu.edu/glossary_search.cfm
(game theory) a stable state of a system that involves several interacting participants in which no participant can gain by a change of strategy as long as all the other participants remain unchanged
http://wordnet.princeton.edu/perl/webwn
In game theory, the Nash equilibrium (named after John Forbes Nash, who proposed it) is a kind of solution concept of a game involving two or more players, where no player has anything to gain by changing only his or her own strategy unilaterally. ...
http://en.wikipedia.org/wiki/Nash equilibrium