1. An individual or organization, including a service provider, that is provided a telecommunications service for which a priority level has been requested or assigned. See also priority level,  request,  service,  service provide,  telecommunications service. 2. In open systems interconnection architecture, an entity in a single open system that makes use of a service via service access points. See also entity,  open system,  opens systems architecture,  open systems interconnection,  service,  service access point. See called service user,  receiving service user,  sending service user.