faa definition on training devices for level of flight simulation
A good way to start is by reviewing what is an approved training device, what is an FAA-approved simulator, and what is a PCATD. At the bottom end of the spectrum is the old ground training devices many of which are now designated as Level One Flight Training Devices (FTD). There are seven levels of FTD's. At the other end of the spectrum are the very complex, state of the art, FAA-approved flight simulators.