ASME Y14.100 : Engineering Drawing Practices
Two or more parts that form a portion of an assembly or a unit replaceable as a whole but having a part or parts that are individually replaceable - e.g., gun mount stand, window sash, recoil mechanism, floating piston, telephone dial, Intermediate Frequency (IF) strip, terminal board with mounted parts.
See Also:
Inseparable Assembly, Assembly, Part, Unit
Alternate Definitions:

DAU - Two or more parts joined to form a unit that is capable of being disassembled and that is only a part of a complete machine, structure, or other article.

PTC - An assembly that is placed within a higher level assembly.