Fitting: from garment to pipe joint
To most people, a fitting is how clothes sit on the body. To a mechanical engineer, it is a standardized connector that joins pipes together.
In everyday speech, a fitting is about apparel. A suit fitting means trying clothes on to check the tailoring. A fitting room is where you do this. The word captures the sense of something shaped to match something else, usually your body.
On the shop floor, a fitting is a discrete, standardized part of a piping or tubing system. It connects two lengths of pipe, or a pipe to a valve, tank, or instrument. Common types include elbows (bends at 45 or 90 degrees), tees (three-way junctions), couplings (straight joins), unions (reversible couplings), and reducers (steps between different pipe diameters). They are mass-produced to published standards such as ANSI or ISO, with exact thread pitches, port sizes, and material specifications. A plumber or pipefitter selects a fitting from a catalog by nominal size and type, installs it with appropriate seals or tape, and trusts it to work.
The term fitting has also spread into electrical work, where a fitting is any standardized component or connector: a light fitting, an outlet fitting, a cable fitting. The principle is the same: mass-produced, interchangeable, and specified by standard.
The metaphor is obvious. Just as a garment fitting makes fabric sit right on your body, a pipe fitting makes two different sections of pipe sit right with each other. The shared root is the verb fit, meaning to be the right size and shape, to match, to connect. In both cases you are achieving a match between two things. But in mechanical work the match must be precise, permanent, and leak-proof, and the fitting itself must be a separate, removable part. Garments are fitted by adjustment of fabric; pipes are fitted by addition of a fitting.
The dictionary entry
fitting
(noun, mechanical engineering)
A (generally small) component or part of a device or machine, especially one that is standardized and replaceable; specifically, a standardized part of a piping system used to connect sections of pipe together, such as a coupling.