Not “someone usually loses when someone wins.” A specific claim: gains and losses cancel out exactly, because the total available is fixed.
Zero-sum is a formal category from game theory: an interaction where the combined payoff to all parties always sums to a fixed constant, so any gain to one side is mathematically another side’s loss. Von Neumann and Morgenstern formalised two-person zero-sum games as the founding case of modern game theory, before later work extended the framework to non-zero-sum and cooperative settings, where both sides can genuinely gain or lose together rather than trading off against each other.
The term is used loosely in everyday language to mean “adversarial” or “competitive” in general, but most real economic and relational situations aren’t zero-sum — value can be created (positive-sum) or destroyed (negative-sum) by how parties interact, not just redistributed between them. Calling a relationship or a negotiation zero-sum is a specific, checkable claim about its structure, not just a description of its tone.
Source: von Neumann, J. & Morgenstern, O., Theory of Games and Economic Behavior, Princeton University Press, 1944; Nash, J.F., “Non-Cooperative Games,” Annals of Mathematics, 1951.