System under test (SUT) refers to a system that is being tested for correct operation. According to ISTQB it is the test object. The term is used mostly in software testing.
A special case of a software system is an application which, when tested, is called an application under test.
The term SUT means also a stage of maturity of the software, because a system test is the successor of integration test in the testing cycle.
References
System under test Wikipedia(Text) CC BY-SA