A simulation is a recreation of something that can be used for many reasons, such as training or entertainment.

One form of simulation is the holoprogram, which recreates people, places and things using holograms and a holodeck. (TNG: "Encounter at Farpoint")

Prior to holodecks, Starfleet Academy had a life-sized recreation of a starship's bridge to train cadets for command situations. One popular simulation was the Kobayashi Maru scenario. (Star Trek II: The Wrath of Khan)