Stock-Keeping unit (SKU)

SKU is a unique identifier for each distinct product and service that can be purchased. SKU use is rooted in data management, enabling the company to systematically track its inventory or product availability, such as in warehouses and retail outlets. They are often assigned and serialized at the merchant level. Each SKU is attached to an item, variant, product line, bundle, service, fee, or attachment. SKUs are often used to refer to different versions of the same product.
Reference:
http://en.wikipedia.org/wiki/Stock-keeping_unit