This quick start says
A tenant is a representation of an organization. It's a dedicated instance of Azure AD that an organization or app developer receives when the organization or app developer creates a relationship with Microsoft-- like signing up for Azure, Microsoft Intune, or Microsoft 365.
The docs here say
An Azure subscription has a trust relationship with Azure Active Directory (Azure AD), which means that the subscription trusts Azure AD to authenticate users, services, and devices. Multiple subscriptions can trust the same Azure AD directory, but each subscription can only trust a single directory.
The terminology link in Elliot's answer explains
A dedicated and trusted instance of Azure AD that's automatically created when your organization signs up for a Microsoft cloud service subscription, such as Microsoft Azure, Microsoft Intune, or Office 365. An Azure tenant represents a single organization.
Azure AD directory
Each Azure tenant has a dedicated and trusted Azure AD directory. The Azure AD directory includes the tenant's users, groups, and apps and is used to perform identity and access management functions for tenant resources.