Single-Tenancy is a concept of software architecture where a single instance of the application software supports only one Tenant company at a time.  This contrasts with Multi-Tenancy where a single instance of software runs on a Software as a Service basis on the vendor's servers, serving multiple client organizations (tenants). 

Single-Tenancy application design became popular during the 1980's when most business software was developed using the Client/Server model.  At the time, there was no publically available network linking software vendors to users and so software had to be delivered on disk and later on CD.

In the Single-Tenancy, Client/Server model the independent software vendor (ISV) makes hundres or thousands of copies of the application, which are delivered to the customer and run on customer's computers.  Due to the existance of different operating systems, versions and service packs coupled with different application package versions and patches the ISV has to face the 'Support grid of death' when it comes to post-sales support and upgrading of Single-Tenant, Client/Server applications installed on customer premises.

Single-Tenancy applications are cheaper to design and build, compared to Multi-tenancy applications and ISV's with Single-Tenancy applications are often reluctant to re-invest in order to embrace the Multi-Tenancy model.  Such ISV's prefer to use either simple hosting and/or virtualization in order to offer pseudo SaaS offerings to the market.  However the pseudo SaaS offerings still contain elements of the support matrix of death and consequently such ISV's will lack the velocity and agility of the pure SaaS/Multi-Tenancy vendors.

For a variety of reasons, fewer applications are being designed and developed using the Single-Tenancy model.  The main reason is that customers are switching to the SaaS model and demonstrate a distinct preference for the nimbler multi-tenancy based SaaS applications.