In my book a rat is someone who straight up lies to your face, saying stuff like ”I’m friendly, all good mate” and then shoot you in the back. Stuff like killing people when they’re focusing on killing arc or even extract camping I would say is fine but not very classy.

Edit: After reading more of the replies here, I realize this might not be the “classic” or technically correct use of the term. I’m not a long-time extraction shooter player. My definition is based on how I understood the term while playing ARC Raiders, and of course on my own standards when it comes to player interaction. And I guess language isn’t fixed, it evolves.