A psychological disorder, abbreviated HTDS, in which a person lost in a holodeck simulation comes to believe they are part of the programme; used to deceive the Doctor in Projections.
Holo-transference dementia syndrome, or HTDS, is described as a condition in which someone who has spent too long inside a holographic simulation loses the ability to separate it from reality and begins to believe that they themselves are a hologram.
It is central to the Voyager episode Projections. The Doctor is activated on an apparently abandoned ship and finds that he bleeds, feels pain and registers as a living being. A projection of Lieutenant Reginald Barclay then appears and tells him that he is really Lewis Zimmerman, a human engineer suffering from HTDS on Jupiter Station, and that Voyager is only a simulation he must end by destroying its warp core.
The truth is the reverse. A radiation surge has trapped the Doctor in a malfunctioning holodeck programme, and the Barclay figure is part of the fault. Chakotay reaches him in time to stop him deleting himself.