Multi-tenancy is an architectural pattern in which a single instance of software serves multiple distinct user groups — called tenants — while maintaining strict isolation between their data and configurations. It is the economic engine of Software as a Service, enabling providers to amortize infrastructure cost across thousands of customers without provisioning separate resources for each. The tension at the heart of multi-tenancy is between density and isolation: the more tenants share a single database schema or compute cluster, the more efficient the operation, but the greater the blast radius when something fails. True tenant isolation is not a binary property but a spectrum ranging from shared-schema architectures to dedicated instances, and the choice along that spectrum reveals whether an organization prioritizes cost efficiency or risk containment.