The relationship of organizational entity to a software installation. Variations on the term indicate whether or not more than one unrelated entity will interact with the same software.
Most CMS are single-tenant, which means a single implementation is designed for use by one organization, either installed on-premise or via SaaS.
Other systems are multi-tenant, which means multiple organizations use the same installation. Necessarily, these systems are hosted by the vendor and provided on a subscription basis in a SaaS model.