A type of accession identifier uniquely identifying a single stockable item, all specimens of which are interchangeable for the purpose intended by the developer when creating the design. This is the number marked on the part, or on the container if the part’s surface or container if the part’s size or usage precludes direct marking.
Note the careful phrasing of the basic definition! It is possible for a single part to be marked with more than one Part Number, as long as each one denotes interchangeability for a distinct purpose in the context of different next-higher assemblies. See, for example, Altered Item Drawing and Selected Item Drawing both of which can result in additional part numbers marked on a single specimen of some item already marked by an OEM.
The notion of “…stocked item…” is also important. This actually means that one-off (unique) designs do not necessarily require Part Numbering (although some of their components might). This notion bakes the noodles of many respected experts in Configuration Management.
Contrast with Model Number. See also the CI Development Cycle (Intermediate).