The first fundamental welfare theorem states that any market in competitive equilibrium will have this property, which applies to scenarios where all consumers have the same marginal rate of substitution for two given goods. A generalization of this 
The first fundamental welfare theorem states that any market in competitive equilibrium will have this property, which applies to scenarios where all consumers have the same marginal rate of substitution for two given goods. A generalization of this property only requires that potential improvements for some are enough to theoretically compensate for the losses of others, and is named for (*) Kaldor and Hicks. Any point lying inside a production possibility frontier represents a system without it, while the frontier's border defines the set of situations where this property holds. For 10 points, name this property of a system where no one can be made better off without making someone else worse off, an "optimality" named for an Italian economist and sociologist.
The first fundamental welfare theorem states that any market in competitive equilibrium will have this property, which applies to scenarios where all consumers have the same marginal rate of substitution for two given goods.
Pareto efficient [or word forms; or Pareto optimality; prompt on just "efficient" or "optimality"]