1. a significant event/object/product that changes the course of things
a game-changer in the industry
This is a great term to use for new technology, for example, or even if your favourite team signs a great new player. If you think things are going to get better - and more difficult for the competition - then you're dealing with a game-changer. This is suitable for social or professional contexts, but you're more likely to hear it in a professional situation.