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]

The Collaborative International Dictionary of English v.0.48:

aqualung \aqualung\ n.
   an apparatus containing compressed air or other oxygen-gas
   mixture, permitting a person to breathe under water; -- also
   called a scuba.

   Syn: scuba
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

scuba
    n 1: a device (trade name Aqua-Lung) that lets divers breathe
         under water; scuba is an acronym for self-contained
         underwater breathing apparatus [syn: aqualung, Aqua-
         Lung, scuba]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "scuba":
   Aqua-Lung, air cylinder, aquascope, artificial respiration,
   aspiration, asthmatic wheeze, bathyscaphe, bathysphere,
   benthoscope, breath, breath of air, breathing, broken wind, cough,
   diving bell, diving boat, diving chamber, diving goggles,
   diving helmet, diving hood, diving mask, diving suit, exhalation,
   expiration, exsufflation, gasp, gulp, hack, hiccup, inhalation,
   inhalator, inspiration, insufflation, iron lung,
   mouth-to-mouth resuscitation, oxygen mask, oxygen tent, pant,
   periscope, puff, respiration, sigh, sneeze, sniff, sniffle, snore,
   snoring, snorkel, snuff, snuffle, sternutation, stertor, submarine,
   suspiration, swim fins, wet suit, wheeze, wind