The CUSTOMER in the Question

The actual customer is the person responsible for the deployed success of the product.

The customer may not be the buyer or even the end user.  They could be in those roles or have an entirely different role.  The key attribute defining the customer is the person who is focused on making sure the goods or services meet the needs of the end-user, whether it is themselves or someone else.  They are responsible for weighing alternatives, including the alternative to do nothing.