What is the difference between an aviation training device (ATD) and a flight training device (FTD)?
14 CFR Part 141, Chapter 41 Flight simulators, flight training devices, and training aids. Requirements for a FTD:


  (1) Be a full-size replica of instruments, equipment panels, and
  controls of an aircraft, or set of aircraft, in an open flight deck
  area or in an enclosed cockpit, including the hardware and software
  for the systems installed that is necessary to simulate the aircraft
  in ground and flight operations;


The important mention is it must be a full-size replica of controls and panels. This doesn't apply for an ATD (see definition in AC 61-136A, appendix 2).

See also:


Sorting the simulators.
FAA withdraws ATD rule.