B CAM

It's the second camera on a shoot, with the first being the A camera. Multi-camera setups are perhaps more commonly used on TV shows (especially sitcoms) than in films, but it's not unusual. Two cameras can be used simultaneously to speed up the shooting of dialogue scenes and save doing multiple setups. For unrepeatable set pieces - stunts, explosions etc. - several cameras are often used.

Hurt Locker I think had 4 cameras at all times. (sorry, one of my favorite productions).

often used in a high risk shot to avoid damaging the typically more expensive A-camera.