Holodeck
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A holodeck is a room capable of projecting a fully-realistic holographic environment through the use of holoemitters, forcefields, replicators and transporter-based technology used for training and/or recreation. Though developed by the Federation well before the mid-24th century, holodecks were not common aboard starships until the launch of the Galaxy-class in the 2360s. Within a decade, nearly every class of Starfleet vessel would have holodecks installed. Other races also make use of holdeck technology, including the Cardassian Union and the Romulan Star Empire. (TNG: "Encounter at Farpoint", "Future Imperfect", DS9: "It's Only a Paper Moon", et. al)
Under normal circumstances, matter created on the holodeck cannot exist outside that environment. Despite this limitation, several holograms have been known to become sentient. Holodecks often have a tendency to malfunction, sometimes creating unique, or even dangerous, situation. (TNG: "11001001", "Elementary, Dear Data", "Ship In a Bottle", "The Big Goodbye", "Emergence"; DS9: "His Way"; VOY: "Bride of Chaotica!", "Worst Case Scenario")
A smaller variant designed for single-occupant use is the holosuite, supposedly invented by Plegg, a Ferengi entrepreneur.
In 2379, Norman Craig and Tracy Carson attempted to take a romantic vacation on the holodeck aboard the USS Pioneer, when suddenly the entire ship's crew disappears and the two of them, along with the assistance of the Emergency Medical Hologram, had to uncover what happened. (Star Trek: Pioneer (STP): "The Vacation")
As of 2385, the Suliban developed very crude holodeck technology to the point where the Romulans had to overhaul the holodecks in the Suliban's cloaking starbase. (RIS Bouteina: "Cloak, baby, Cloak!")
