Ghost conditioning is a powerful form of hypnosis which restricts the use of ghost's psionic abilities. It is inflicted on ghosts by the Ghost Program. In addition to the psychological blocks, it also involves powerful chemicals.
This conditioning was so powerful that even after Sarah Kerrigan was infested, she did not immediately lose this psychic hindrance. Kerrigan was able to overcome the conditioning by raiding a science vessel, the Amerigo, and reading the old Ghost Program files.