The Collaborative International Dictionary of English v.0.48:

Self-contained Underwater Breathing Apparatus \Self`-con*tained"
Underwater Breathing Apparatus\, n.
   A portable device to allow divers to breathe while under
   water, consisting of one or two tanks of compressed air which
   are strapped onto the back of the diver, and are connected by
   tubing to a mouthpiece through which the diver receives the
   air from the tanks at rate adjustable by a valve; -- called
   also SCUBA, SCUBA gear, or SCUBA apparatus.
   [PJC]