A scuba set is an independent breathing set that provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving. It is much used for sport diving and some sorts of work diving.
The word SCUBA is an acronym for Self-Contained Underwater Breathing Apparatus. These initials originated in 1939 in the US Navy to refer to US military diver`s rebreather sets. As with radar, the acronym has become so familiar that it is often not capitalized and is treated as an ordinary word; for example, it has been taken into the Welsh language as "sgwba".
Modern scuba sets are of two types:
open-circuit (In Europe, but not the U.S., often called an "aqualung", see Aqua-Lung™, first invented by Jacques Yves Cousteau and Emile Gagnan). Here the diver breathes in from the set and out to waste. This type of equipment is relatively simple, making it cheaper and more reliable. The two-hose design originally used was the one designed by Cousteau and Gagnon. The single-hose design generally used today was invented in Australia by Ted Eldred.
closed-circuit/semi-closed circuit (also referred to as a rebreather). Here the diver breathes in from the set, and out back into the set where the exhaled gas is processed to make it fit to breathe again. These existed before the open-ciruit sets and still exist today, but they are used less than open-circuit sets.
Both types of scuba provide a means of supplying air or other breathing gas, nearly always from a high pressure diving cylinder, and a harness to strap it to the diver`s body. Most open-circuit scuba and some rebreathers have a demand regulator to control the supply of breathing gas. Some rebreathers only have a constant-flow regulator like in blowtorches. Some divers use the word "scuba" to mean open-circuit sets only.