'''System under test''' ('''SUT''') refers to a [[software system|system]] that is being tested for correct operation. According to [[International Software Testing Qualifications Board|ISTQB]] it is the test object.<ref>{{cite web|url=http://glossary.istqb.org/search/test%20object|title=ISTQB Glossary|website=glossary.istqb.org|accessdate=2018-02-05}}</ref><ref>{{cite web|url=https://robots.thoughtbot.com/don-t-stub-the-system-under-test|title=Don't Stub the System Under Test|website=robots.thoughtbot.com|date=22 November 2013 |accessdate=2018-02-05}}</ref><ref>http{{cite web|url=https://www.itwissen.info/definition/lexikon/SUT-system-under-test.html</ref>|title=SUT The(system termunder is used mostly in [[software testing]]test)|website=www.itwissen.info|language=de|accessdate=2021-02-24}}</ref>
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.
A special case of a software system is an [[application software|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.{{clarify |date=December 2017 |reason= maturity and testing cycle appears to refer to a phased-approach to SW development}}
