In game theory (and economic theory) a zero-sum game is a situation where one persons (or entities) gain has a corresponding loss from another person (or entity).
Initech employee 1: "Hey, everyone took big pieces of the cake now Milton doesn't have one."
Initech employee 2: "Cake is a zero sum game, man."
First Person: Hey look at this funny cat video
SecondP: OMG this cat is awesome
ThirdP: Is this meme zero?
FirstP: No, this is photoshopped in, meme zero is from a youtube video