Holodeck

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 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. (,, 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. 

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 (NCC-1084), 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. (🇸🇹: "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.

The Lyrans also possessed holodeck technology. The Lyran Starfleet survivors of the Battle of Krypton were assigned all the holodecks of Starbase 47 (Lyran) for a day.

Also, the Hydran Keepership, in a never-ending craze for greater maximum evacuation capacity, sent the RIS to search for an artifact on Krant that supposedly allowed infinite evacuation capability. It turned out that the artifact was a technique called Russian-doll holodeck where holodecks are made within holodecks, ad infinitum.

Malachorians, as well as a variety of other species, operated holodeck theaters where a group of people could use a common holodeck program for a fee.