A stock-keeping unit or SKU (pronounced either as an acronym, /ˈskyuː/, or as an initialism, /ˌɛsˌkeɪˈyuː/) 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 not always associated with actual physical items, but are more appropriately billable entities. Extended warranties, delivery fees, and installation fees are not physical, but have SKUs because they are billable. All merchants using the SKU method will have their own approach to assigning the SKU system based on regional or national corporate data storage and retrieval strategies. SKU tracking varies from other product tracking methods which are controlled by a wider body of regulations stemming from manufacturers or possibly third-party regulations.

Successful inventory management systems assign a unique SKU for each product and also for its variants, such as different versions or models of product or different bundled packages including a number of related products. This allows merchants to track, for instance, whether blue shirts are selling better than green shirts. Other entity tracking methods, with varying regulations, are Universal Product Code (UPC), European Article Number (EAN), Global Trade Item Number (GTIN) and Australian Product Number (APN).