Multi-tenant software applications serve different organizations from a single instance and help to save development, maintenance, and administration costs. The architectural concepts of these applications and their relation to emerging platform-as-a-service (PaaS) environments are still not well understood, so that it is hard for many developers to design and implement such an application. Existing attempts at a structured documentation of the underlying concepts are either technology-specific or restricted to certain details. We propose documenting the concepts as a new architectural style. This paper initially describes the architectural properties, elements, views, and constraints of this style. We illustrate how the architectural elements are implemented in current PaaS environments, such as Force.com, Windows Azure, and Google App Engine.