Plain English Meaning
A ground-based machine that copies the cockpit and behaviour of a real aircraft closely enough that the FAA lets you do some of your training and checkrides in it instead of in the airplane.
Full Definition
Ground-based training equipment that replicates the instruments, controls, and systems of a specific aircraft or class of aircraft, used to teach procedures and develop skills without leaving the ground. A flight training device (FTD) is approved by the FAA to a specific qualification level, and certain training and checking tasks completed in an FTD can count toward pilot certification, ratings, and currency requirements.