Software Architect's Handbook

Controllability

Controllability represents the level to which it is possible to control the state of the component being tested. The component being tested is sometimes referred to as the system under test (SUT) or the component under test (CUT). Controlling the state of a component involves being able to dictate the inputs of the component as well as the level to which those inputs exercise its capabilities.

When designing components, you should seek to increase their controllability, as this increases testability.