Holo-transference dementia syndrome, or HTDS, was a medical condition in which an individual became overly involved in a holographic simulation and thereby began to lose his/her sense of identity. HTDS ultimately resulted in the person thinking they were an actual part of the simulation rather than a living, physical individual.
In a delusion caused by a kinoplasmic radiation surge in 2371, The Doctor was told by Reginald Barclay that he was suffering from HTDS, that he was in fact Lewis Zimmerman, and that USS Voyager was a hologram. (VOY: "Projections")