A gamechanger is the thing or person that causes a huge shift — the new tool, the new rule, the new player that makes everyone do things differently afterwards. It is the noun that matches the adjective game-changing. You will hear it in business English, sports commentary, and tech writing. In careful writing it is very often game-changer, with a hyphen; the one-word form gamechanger is the same idea, just packed tighter. In Taiwan, office people often keep the English words as they are. The tone is informal to neutral: lively in a meeting, a little casual in a formal essay.