Piping and plumbing fitting

A fitting or adapter is used in pipe systems to connect sections of pipe (designated by nominal size, with greater tolerances of variance) or tube (designated by actual size, with lower tolerance for variance), adapt to different sizes or shapes, and for other purposes such as regulating (or measuring) fluid flow. These fittings can manipulate the conveyance of fluids such as water for potatory, irrigational, sanitary, and refrigerative purposes, or other liquids like gas, petroleum, and liquid waste.

Source: Wikipedia — Piping and plumbing fitting (CC BY-SA 4.0)