ASME Y14.100 : Engineering Drawing Practices
One item, or two or more items joined together, that is not normally subject to disassembly without destruction or impairment of designed use (e.g., transistor, composition resistor, screw, transformer, and gear).
See Also:
Assembly, Subassembly, Item, Inseparable Part Model
Alternate Definitions:

Action Engineering - A collection of two or more parts that are only disassembled by the destruction of one or the other parts. These may be represented as a part or an assembly model.