Multi-tenancy is a SaaS architecture where a single instance of the software serves multiple customers (tenants), with data isolation between them. Most SaaS products are multi-tenant — one application layer handles all customers, separating their data in the database rather than running separate servers for each.
The opposite is single-tenancy: each customer gets a dedicated instance. Multi-tenancy is far more cost-efficient to operate, but requires rigorous data isolation to prevent cross-tenant data leakage. Security-sensitive enterprise customers sometimes demand single-tenant or "private cloud" deployments, typically at a significant price premium.