A dedicated instance of Microsoft Entra ID (formerly Azure Active Directory) that represents an organization. It holds the users, groups, and applications. An organization has a single tenant but can have multiple subscriptions.
The top layer of the identity hierarchy in AZ-900 and AZ-104. The tenant is the directory; the subscription is the billing unit. Question: can a subscription have multiple tenants? No, but a tenant can have multiple subscriptions.
Azure tenant (Microsoft Entra ID) is a concept that can show up on the exam. Check the full guide to see it in context.