I like to call it the 'system under test' / SUT. Then I can go on to say a unit test is a test that's fast and if it fails points to a specific problem in the SUT. Smaller SUTs obviously help satisfy both. If setting up collaborators in a bigger SUT leads to either unacceptable runtimes or too much brittleness from many things being able to go wrong, it's a signal you're not getting the value of unit testing in that place. (The test itself may or may not be valuable, other types of testing have their own value propositions.)