Design under test (DUT)
Beginner
The part of the chip that a test is checking.
Novice
The piece of the design being tested, placed inside a testbench. It can be one small block or the whole chip. Also called the DUV, design under verification.
Expert
Its boundary decides how easily a test can steer it (controllability) and see the effects (observability). Small DUTs are easier to drive into rare cases and to check, which is why most random and formal effort happens on individual blocks.