Scuba diving is using self-contained breathing equipment to stay underwater for periods of time greater than human breath-holding ability allows. The diver carries all equipment necessary for diving and is not reliant upon equipment elsewhere (e.g. on the surface) to supply breathing gas or other support during the dive. The diver swims underwater using fins attached to the feet. Some divers also move around with the assistance of a DPV (Diver Propulsion Vehicle), commonly referred to as a "scooter", or by using surface-tethered devices called sleds, which are pulled by a boat.

The word 'SCUBA' is an acronym for "Self Contained Underwater Breathing Apparatus", but it has become acceptable to refer to 'scuba equipment' or 'scuba apparatus'. Scuba has become an anacronym in popular culture. The term SCUBA in common usage usually means open-circuit equipment in which gas (usually air) is breathed from a tank of compressed gas and then exhaled into the water, usually in the line of kit development started by Emile Gagnan and Jacques-Yves Cousteau. However, rebreathers (both semi-closed circuit and closed circuit) are also self-contained systems (as opposed to surface-supplied systems) and could be classed as SCUBA. The term SCUBA arose during World War II and originally referred to USA combat frogmen's oxygen rebreathers, developed by Dr. Christian Lambertsen for underwater warfare.

Scuba diving is still evolving, but general classifications have grown up to describe various diving activities. These classifications include, but are not limited to:

• recreational diving

• public safety diving

• technical diving (also called Tech Divers)

  • Cave diving

  • Deep diving

  • Ice diving

  • Wreck diving

• military diving: this includes combat divers and armed forces work divers.

• commercial diving.

• Scientific diving.

Within recreational diving there are those who are considered professional divers, because they maintain a professional standard of training and skills (and must, in theory, according to the laws of the area, carry professional liability insurance).

Some consider technical diving to be a subset of recreational diving, but others separate it out due to the extensively different training equipment and knowledge needed for technical dives.

Public safety diving and military diving might likewise be classified as commercial diving because public safety divers and military divers make a living from their pursuit of diving. However, public safety divers (police or rescue) and military divers have a different mission from the typical commercial diver.

Scientific diving is used by marine scientists (including diving marine biologists), as a tool for collecting their research data.