A fitting is used in pipe systems to connect straight pipe or tubing sections, adapt to different sizes or shapes and for other purposes, such as regulating {or measuring) fluid flow. "Plumbing" is generally used to describe the conveyance of water, gas, or liquid waste in domestic or commercial environments; "piping" is often used to describe the high-performance (high-pressure, high-flow, high-temperature or hazardous-material) conveyance of fluids in specialized applications. "Tubing" is sometimes used for lighter-weight piping, especially those flexible enough to be supplied in coiled form.

Fittings (especially uncommon types) require money, time, materials and tools to install, and are an important part of piping and plumbingsystems. Valves are technically fittings, but are usually discussed separately.