A part of a system that encapsulates behavior, exposes a set of interfaces, and packages other model elements.

From the outside, a subsystem is a single design model element that collaborates with other model elements to fulfill its responsibilities. The externally visible interfaces and their behavior is referred to as the subsystem specification.

On the inside, a subsystem is a collection of model elements (design classes and other subsystems) that realize the interfaces and behavior of the subsystem specification. This is referred to as the subsystem realization.