What is Universal Product Code?

Legal Definition
The Universal Product Code (UPC) is a barcode symbology that is widely used in the United States, Canada, United Kingdom, Australia, New Zealand, and in other countries for tracking trade items in stores.

UPC (technically refers to UPC-A) consists of 12 numerical digits, that are uniquely assigned to each trade item. Along with the related EAN barcode, the UPC is the barcode mainly used for scanning of trade items at the point of sale, per GS1 specifications. UPC data structures are a component of GTINs and follow the global GS1 specification, which is based on international standards. But some retailers (clothing, furniture) do not use the GS1 system (rather other barcode symbologies or article number systems). On the other hand, some retailers use the EAN/UPC barcode symbology, but without using a GTIN (for products, brands, sold at such retailers only).
-- Wikipedia
Legal Definition
Barcode of 12 digits assigned to every product sold at a retail store.