Game-changing describes something that does not just improve a situation a little, but turns the whole thing onto a new path — as if the rules of the game itself had changed. People use it a lot in news, business, and tech talk: a game-changing idea, a game-changing invention, a game-changing court decision. It comes from sport: a play so big that the rest of the match feels different. In careful English it is usually written with a hyphen, as an adjective before a noun. Though actually, people now throw it around rather loosely, the way they throw around ‘amazing’, so a small update sometimes gets called game-changing when it is only handy. The tone is modern and a bit journalistic. The ‘game’ here is the contest, or the way a thing is done — not wild animals, and not only video games.