System under test (SUT) refers to a system that is being tested for correct operation. According to ISTQB it is the test object.

From a unit testing perspective, the system under test represents all of the classes in a test that are not predefined pieces of code like stubs or even mocks. Each one of this can have its own configuration (a name and a version), making it scalable for a series of tests to get more and more precise, according to the quantity of quality of the system in test.