A simulation was a recreation of something that could be used for many reasons, such as training or entertainment. The order of events in a simulation was called a simulation sequence.
One form of simulation was the holoprogram, which recreated 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, Star Trek)
Certain recreation rooms were capable of holodeck-like simulations of environments. By 2270, the USS Enterprise was outfitted with such a room. This rec room was capable of creating pre-programmed illusions using holographic scenes, simulated weather including wind and temperature, and sounds played from audio tapes. The scenes it created included an 18th centuryEuropean hedge maze, forests, blizzards, and a beach with seagulls. Even though the scenery extended beyond the room, one would still be able to walk up to and touch the walls during a simulation. (TAS: "The Practical Joker")
Tri-phasic emitters were a Romulanweapons technology which allowed a vessel to simulate the weapons fire of alienspacecraft.
Romulans also possessed multispectral emitters which allowed a ship to create the illusion of being other ships. The Romulans used this ability to change appearance in an effort to create tension between the regional powers. (ENT: "Babel One", "United")
Echo displacement was a technique to create decoy images on an enemy's sensors, using a starship's deflector grid. (VOY: "Basics, Part I")
Terraspheres were large space stations built by Species 8472. One such station encountered by the USS Voyager in 2375 consisted of a domed chamber in which holographic technology and particle synthesis were used to create a full-scale replica of Starfleet Headquarters and the surrounding area of San Francisco on Earth. (VOY: "In the Flesh")
Starfleet sometimes used simulator holopods to test whole crews on a variety of situations experienced by other starships, such as Shari yn Yem's visit to the USS Cerritos in 2381. (LD: "I, Excretus")
