A flight simulator is a device that artificially re-creates aircraft flight

Flight

Flight is the process by which an object moves either through an atmosphere or beyond it by generating lift or propulsive thrust, or aerostatically using buoyancy, or by simple ballistic movement....

 and various aspects of the flight environment. This includes the equations that govern how aircraft fly, how they react to applications of their controls and other aircraft systems, and how they react to the external environment such as air density, turbulence, cloud, precipitation, etc. Flight simulation is used for a variety of reasons, including flight training

Flight training

Flight training is a course of study used when learning to pilot an aircraft. The overall purpose of primary and intermediate flight training is the acquisition and honing of basic airmanship skills....

 (mainly of pilots), for the design and development of the aircraft itself, and for research into aircraft characteristics, control handling qualities, and so forth.

Flight simulations have varying degrees of hardware, modelling detail and realism that depend on their purpose.