defector

A defector is someone who leaves their job or organization to join another one. It can also mean leaving a country to live somewhere else.

Imagine working for a company and feeling unhappy with the way things are run. You might quit your job and start fresh with a new employer - that's a common sense of what it means to be a defector in this context. Now, if you were a spy or an official in a government, you could be a defector too by leaving your country to defect to another one. This is often used in the news when someone high up in a government or organization leaves to join another group.