A category of unit with a unique combination of form, fit, and function (i.e., unique components held in stock). To illustrate: If two items are indistinguishable to the customer, or if any distinguishing characteristics visible to the customer are not important to the customer so that the customer believes the two items to be the same, these two items are part of the same SKU.
As a further illustration: consider a computer company that allows customers to configure a complete computer from a selection of standard components. For example, they can choose from three keyboards, three monitors, and three CPUs. Customers may also individually buy keyboards, monitors, and CPUs. If the stock were held at the configuration component level, the company would have nine SKUs. If the company stocks at the component level, the company would have 36 SKUs. (9 component SKUs + 3x3x3 configured product SKUs.) If, as part of a promotional campaign, the company also specially packaged the products, the company would have a total of 72 SKUs.