The concept of multitenancy actually dates back to the 1960s, when companies rented time on mainframes, which were rare and expensive. Back then it was called time sharing. Multiple customers could access the same apps at the same time, a feat only mainframes could do.
Starting in the 1990s, application service providers (ASPs) hosted applications on behalf of their customers and like mainframes, the same apps were made available to multiple customers. Learn more about multi-tenant architecture now.