Defection is when you leave your own side — your country, your party, your army, your group — and go over to the other side. It is a strong word. It usually means you have changed sides in a serious fight, not that you simply changed jobs. Newspapers use it for spies, soldiers, and politicians who run to the enemy. In sport writing, people sometimes stretch it for a player who leaves one club for a rival, though that is looser talk. The person who does it is a defector; the verb is to defect. People on the side you left may call it betrayal; news writing often stays with defection, which sounds more like a fact than a moral judgement.