Schmiernippel

A fitting is a connector that joins pipes or hoses, or changes the direction or size of a fluid line.

Elbows, unions, couplings, and nipples link line sections and seal the joint so fluid or air passes without leaking, often allowing the line to be disconnected for service.

Fittings are used in the fuel, air/brake, cooling, and hydraulic lines. The correct fitting matches the thread or connection type and the line size.