Glossary
Tenant
A tenant is your organisation's isolated instance of Microsoft 365: its users, its data, its settings and its domain. Everything in SharePoint, Teams and Entra ID belongs to exactly one tenant, and nothing crosses between them without an explicit configuration.
It is the unit almost everything is scoped and licensed by. A package is deployed once per tenant; an App Catalog is one per tenant; SaaS products in this space are usually priced per tenant rather than per user.
It is also the boundary that matters in a security conversation: “the data stays in your tenant” is a meaningful statement, because it means the data never left the isolation boundary Microsoft maintains for your organisation.