Question

I see many people talking about the term SUT, but do not understand why they use that term.

SUT is what you want to test?

Where does this term come from and what does it mean?

For example in this test, what is my SUT?

[TestMethod]
public void UsersAction_should_return_IndexAction()
{
    const long id = 1;

    UsersViewModel viewModel = new UsersViewModel()
    {
        SelectedUsers = new long[] { 1, 2, 3, 4 }
    };

    ActionResult result = _controller.Users(id, viewModel);

    result.AssertActionRedirect().ToAction("Index");
}

Answer 1

The System Under Test (SUT) from a Unit Testing perspective represents all of the actors (i.e one or more classes) in a test that are not mocks or stubs. In your example that would be the controller.