A product and its variants passing through similar processing steps and common equipment just prior to shipment to the customer. The significance of product families for lean thinkers is that they are the unit of analysis for value-stream maps, which are defined from the most downstream step just before the customer.
Note that product families can be defined from the standpoint of any customer along an extended value stream, ranging from the ultimate customer (the end consumer) to intermediate customers within the production process.
For example: In a power tools business, a product family might be defined as medium-sized electric drills utilizing a common chassis and passing through a common assembly cell as the last manufacturing step before shipment directly to end consumers.
Alternatively, the product family might be defined as the drive motor and its variants assembled in a common cell just prior to shipment to the drill manufacturer customer.
Or a product family might be defined as the drive motor stator and its variants going through a common manufacturing process just prior to shipment to the drive motor customer.