A diving regulator is a pressure regulator used in scuba or surface supplied diving equipment that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases. The gas may be supplied from a cylinder worn by the diver (as in a scuba set), or via a hose from a compressor or a bank of cylinders on the surface (as in surface-supplied diving). A gas pressure regulator has one or more valves in series, which reduces pressure from the source in a controlled manner, lowering pressure at each stage.
The terms "regulator" and "demand valve" are often used interchangeably, but a demand valve is the part of a regulator that delivers gas only while the diver is breathing in and reduces the gas pressure to ambient. In single hose regulators, it is part of the second stage held in the diver's mouth by a mouthpiece. In double hose regulators it is part of the regulator attached to the cylinder.