(covers information from several alternate timelines)
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.
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 century European 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")
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")
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")