An Acceptable Use Policy (AUP) defines what users are permitted and prohibited from doing on an organization's systems, including website access. Accessing unauthorized sites is a common violation of the AUP.
Reference:
CompTIA Security+ SY0-701 Official Study Guide, Domain 5.1: "AUPs outline what is and is not acceptable use of organizational resources." Exam Objectives 5.1: "Explain the importance of organizational security policies, standards, and frameworks."