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.