1. An assembly, or part thereof, that is essential to the operation of some larger assembly and is an immediate subdivision of the assembly to which it belongs.
Note: For example, a radio receiver may be a component of a complete radio set consisting of a combined transmitter-receiver, i.e., a transceiver. The same radio receiver could also be a subsystem of the combined transmitter-receiver, in which case the IF amplifier section would be a component of the receiver but not of the radio set. Similarly, within the IF amplifier section, items, such as resistors, capacitors, vacuum tubes, and transistors, are components of that section.
2. In logistics, a part, or combination of parts having a specified function, that can only be installed or replaced as an entity. .
3. In material, an assembly or any combination of parts, subassemblies, and assemblies mounted together in manufacture, assembly, maintenance, or rebuild.
Source of 1-3: from Federal Standard 1037C and from MIL-STD-188 and from a 1994 draft revision of the Department of Defense Dictionary of Military and Associated Terms and from the Department of Defense Dictionary of Military and Associated Terms
4. In software, the word has an emerging meaning generalizing the idea of a software pattern, software object, software framework and software architecture. A software component can be any of these.
See also: component software theory, software component.
5. In thermodynamics, a component is a chemically distinct constituent of a phase of a system.