In welfare economics or choice theory, we are looking for "best" outcomes. An outcome is called Pareto efficient or Pareto optimal if there are no ways left to make one person better-off, unless we are willing to make some other person worse-off.
The Pareto front consists of all Pareto-efficient situations.
Canonical post: Being the (Pareto) Best in the World
This tag is about articles explaining or exploring different aspects of this concept.