Depth

Technical dives may be defined as being either dives to depths deeper than 130 feet / 40 meters or dives in an overhead environment with no direct access to the surface or natural light. Such environments may include fresh and saltwater caves and the interior of shipwrecks. In many cases, technical dives also include planned decompression carried out over a number of stages during a controlled ascent to the surface at the end of the dive.

Stops

Technical dives may alternatively be defined as dives with durations long enough to require mandatory decompression stops, which may optionally be performed using enriched oxygen breathing gas mixtures such as nitrox or pure oxygen. This definition is derived from the fact that metabolically inert gases, such as nitrogen and helium, in the diver's breathing gas are absorbed into body tissues when breathed under high pressure. These dissolved gases must be allowed to release gradually from body tissues to prevent decompression sickness or the bends. This form of diving implies a much larger reliance on redundancy and training since it is no longer physiologically safe to make a direct ascent to the surface in the case of any problems underwater.

Mixes

Technical dives may also be defined as being to depths requiring the use of breathing gas mixtures other than air such as trimix, heliox, and heliair. This definition is derived from the fact that breathing a mixture with the same oxygen concentration as is found in air (roughly 21%) at depths greater than 180 feet / 55 meters results in a very rapidly increasing risk of severe symptoms of oxygen toxicity. Increasing depth also causes air to become narcotic and results in impairing divers ability to react or think clearly (see Nitrogen narcosis). By adding helium to the breathing mix divers can reduce the narcosis. They can also lower the level of oxygen in the mix to reduce the danger of oxygen toxicity. Once the oxygen is reduced below 16% the mix is known as a hypoxic mix as it doesn't contain enough oxygen to be used safely at the surface.

Ability to ascend

Technical dives often refer to dives with a ceiling prohibiting a direct ascent to the surface: it can either be a mandatory stop (decompression obligation) or a physical ceiling:

• Cave diving - diving into a cave system.
• Ice diving - diving under ice.
• Wreck diving - diving inside a shipwreck.

Equipment

Technical divers may also use various forms of less common diving equipment to accomplish their goals. Typically technical dives involve significantly longer durations than average recreational scuba dives. Technical divers therefore increase their supply of available breathing gas by either connecting multiple high capacity diving cylinders and/or by using a rebreather. The technical diver may also carry additional cylinders, known as stage bottles, to ensure adequate breathing gas supply for decompression with a reserve for  bail out  in case of failure of their primary breathing gas.