Illusion
An illusion was a pseudo-reality created by natural and/or artificial means. For an individual experiencing the illusion, it could be as real as the thing in reality and just as hard to dismiss. The illusion was based on an individual's thoughts, memories, experiences, and dreams. 

Some telepathic species had the natural capacity at creating an illusion for an individual. This act of creation could have a therapeutic effect on an individual who was physically incapacitated. The danger for the telepath was that they could become lost in the illusion, feeling the emotions of the individual, and, over time, become addicted to the experience. As the telepath lived and relived the illusory life of the individual, he or she would gradually give up the ability to travel, to build, and to create. The danger for the individual was that he or she may learn how to create illusions and, potentially, travel upon the same path as the telepath. ()

Certain recreation rooms were capable of holodeck-like simulation of environments. By 2270, the  was outfitted with such a room. This rec room was capable of creating pre-programmed illusions using holographic scenes, simulated weather including wind and temperature, and sounds played from audio tapes. The scenes it created included an 18th century European hedge maze, forests, blizzards, and a  with seagulls. Even though the scenery extended beyond the room, one would still be able to walk up to and touch the walls during a simulation. ()

In 2365, the  was attacked by the Borg after being catapulted into their space by Q. The attack resulted in the deaths of 18 of the vessel's crew and Captain Jean-Luc Picard pleaded with Q to tell him that it was one of his illusions. Q, however, responded that it was as real as their so-called life got and vanished. ()

External link 
 
Psychology