Off-screen or Off-camera. This is the abbreviation sometimes seen next to the CHARACTER'S name before certain bits of dialog. Basically, it means the writer specifically wants the voice to come from somewhere unseen. For example, in
The Truman Show, when Meryl calls to Truman, who's in the bathroom, her dialog is marked with an (O.S.). She's in the scene, but she's not on camera, so she's Off-screen or Off-Camera.