ASME Y14.100 : Engineering Drawing Practices
A number of parts or combination thereof that are joined together to perform a specific function and subject to disassembly without degradation of any of the parts (e.g., power shovel-front, fan assembly, audio- frequency amplifier). NOTE: The distinction between an assembly and a subassembly is determined by individual application. An assembly in one instance may be a subassembly in another instance where it forms a portion of a higher assembly.
See Also:
Inseparable Assembly, Subassembly, Part
Alternate Definitions:

QIF - a number of parts or combination thereof that are joined together to perform a specific function and subject to disassembly without degradation of any of the parts

Note 1 to entry: In QIF, assembly means the design of an assembly, and a physical instance of an assembly is called an actual component.