An architecture where one deployment serves many customer organisations, with their data strictly isolated from each other. The alternative is a separate installation per client.
It is what makes SaaS economics work: one system to update, monitor, and scale instead of fifty. The critical requirement is isolation, because a single missing filter exposes one client's data to another.
We enforce tenant isolation at the data layer rather than relying on application code to remember it, test cross-tenant access explicitly, and support per-tenant configuration without forking the codebase.