In the Star Trek universe, the holodeck is a chamber that
can be programmed to create ultra-convincing replicas of other environments.
A holodeck is able to synthesize actual, solid objects, albeit ones that
can't exist outside the holodeck. These are primarily used for recreation,
but also for simulation and training purposes. By the time of Next Generation,
Voyager, and Deep Space 9, characters have immersed themselves in interactive
adventures called holonovels.
OF STAR TREK