Holodeck

A holodeck is a room that is made up of a hologrid that enable holographic projections to be created inside the room.

History
The origins of the holodeck go back to the beginning of humanity's mission into space. In 2151, the Enterprise (NX-01) encountered a Xyrillian vessel that had encountered engine problems. While aboard the Xyrillian ship, the Enterprise crew discovered a holographic chamber aboard which enabled the Xyrillians to program any environment they desired. A holographic chamber was later installed aboard a Klingon battle cruiser as a bargain for the Xyrillians' lives. (ENT episode: "Unexpected").

By the late 23rd century, Starfleet had reached a reasonable level of holographic projection and rec rooms aboard Constitution-class starships came equipped with holographic projectors. (TAS episode: "The Practical Joker"; and TOS comics: "The Final Voyage" and "Mortal Gods").