"The existence of multiple clients sharing resources (services or applications) on distinct physical hardware. Due to the on-demand nature of the cloud, most services are multi-tenant. When you use software like Trello, a popular task tracking tool, your account and settings are shared on a network that is also being used by millions of other users. Even though you can only see your own data, due to multi-tenancy, Trello is able to achieve economies of scale by being able to host everyone's data together, but isolating them such that data is private only to you. This is a fundamental aspect of SaaS products, which are enabled by cloud technologies, ie. hosting databases and storage in the public cloud."
in other words
The cloud version of an apartment building - one building, many tenants.