From the perspective of the service provider, what is the person or group who defines or and agrees their service targets known as?

A User is the person who uses services day-to-day but does not define or formally agree on service targets with the provider.

ITIL explicitly defines a Customer as the role that defines the requirements for a service and takes responsibility for agreed service outcomes from the service provider's perspective. Customers negotiate and agree on service targets documented in Service Level Agreements (SLAs), distinguishing them from users who simply consume the service.

A Supplier is an external third party that provides goods or services to the IT organization, not a party that defines service targets with the provider.

Administrator is not a recognized ITIL stakeholder role in this context and has no authority to define or agree on service targets.