What is a Zero-Sum Game? 

The term Zero-Sum Game is generally used in game theory to refer to a situation in which any gains (such as profits or benefits) accumulated by one participant is offset by the accumulated losses of one or more other participants.

Basically, if one person acquires value, another person losses value in the game. The total value in the game is not expanded. 

This term is commonly used in economics theory and negotiation practice. 

In negotiations, distributive negotiations are generally considered zero-sum. Integrative negotiations seek positive-sum opportunities - also called non-zero-sum opportunities or win-win situations.

In terms of economic theory, capitalist tend to view national economies a non-zero-sum game (economic activity creates additional value through increased economic activity that spreads about and benefits everybody - though not equally).

Socialists, on the other hand, generally view national economies as a zero-sum game (if A grows $1 richer, this value was at the expense of another participant in the economy).