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A set of decisions "x/y" (meaning that participant A chooses "x" while participant B chooses "y") is called Pareto optimal if there is '''no''' other state, other participants' concurrent actions and everybody's prior actions remaining the same, in which at least one participant can improve his own outcome.

A set of decisions "x/y" (meaning that participant A chooses "x" while participant B chooses "y") is called Pareto optimal if there is '''no''' other state, other participants' concurrent actions and everybody's prior actions remaining the same, in which at least one participant can improve his own outcome.

If a participant can improve his outcome, other participants' concurrent actions and everybody's prior actions remaining the same, the new decision set '''Pareto dominates''' the old set.  When there are no more decision sets that ''Pareto dominate'' the current set, the current set is said to be ''Pareto optimal''.

If a participant can improve his outcome, other participants' concurrent actions and everybody's prior actions remaining the same, the new decision set '''Pareto dominates''' the old set.  When there are no more decision sets that ''Pareto dominate'' the current set, the current set is said to be '''Pareto optimal'''.

==An example - Prisoner's dilemna==

==An example - Prisoner's dilemna==