Holodeck

Holodecks were a form of holographic technology used on Starfleet starships in a parallel universe. (COMIC: Assimilation²)

In an attempt to visit San Francisco, the Doctor's TARDIS accidentally took the Eleventh Doctor, Amy Pond and Rory Williams to the holodeck of the USS Enterprise (NCC-1701-D). At the time, the holodeck was running the program Picard Dixon Hill Seven, set within a simulation of San Francisco. (COMIC: Assimilation²)

In the main universe, they were a fiction seen in the Star Trek franchise. Izzy Sinclair, a longtime fan of the show, once claimed to have travelled with the Eighth Doctor despite his ship not having a holodeck because "beggars [couldn't be] choosers". (AUDIO: Izzy's Story)

Nardole compared the Shadow World to "the holodeck on Star Trek", shortly before he discovered he was part of the Shadow World himself. (TV: Extremis)