self-contained underwater breathing apparatus

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.