[[stage left]], [[stage right]]

I didn't add the definitions, only the glosses, but to my understanding both are valid uses. The very popular phrase from American cartoons "exit, stage left" is the area offstage to the left from the performers perspective. When a performer moves to the left or right on the stage they are also moving "stage left" or "stage right", compare to "upstage" and "downstage". As for relativity, I think it is relative to the performers previous position unless otherwise stated, similar to cardinal directions. I suppose it would be possible to "move stage left of Tim" or similar, but I have no knowledge of that use.