Pareto Optimal

An outcome is said to be Pareto optimal (or Pareto efficient) if there is no other outcome that makes one agent better off without making another agent worse off.

If an outcome is Pareto optimal, then at least one agent will be reluctant to move away from it (because this agent will be worse off).

If an outcome $\omega$ is not Pareto optimal, then there is another outcome ${\omega }^{\prime }$ that makes everyone as happy, if not happier, than $\omega$.

“Reasonable” agents would agree to move to ${\omega }^{\prime }$ from $\omega$ if $\omega$ is not Pareto optimal and ${\omega }^{\prime }$ is.