Hallucination. A state produced sometimes by physiological disorders, sometimes by mediumship, and at others by drunkenness. But the cause that produces the visions has to be sought deeper than physiology. All such visions, especially when produced through mediumship, are preceded by a relaxation of the nervous system, in variably generating an abnormal magnetic condition which attracts to the sufferer waves of astral light. It is the latter that furnishes the various hallucinations. These, however, are not always what physicians would make them, empty, and unreal dreams. No one can see that which does not exist—i.e., which is not impressed—in or on the astral waves. A Seer may, however, perceive objects and scenes (whether past, present, or future) which have no relation whatever to himself, and also perceive several things entirely disconnected with each other at one and the same time, thus producing the most grotesque and absurd combinations. Both drunkard and Seer, medium and Adept, see their respective visions in the Astral Light; but while the drunkard, the madman, and the untrained medium, or one suffering from brain-fever, see, because they cannot help it, and evoke the jumbled visions unconsciously to themselves, the Adept and the trained Seer have the choice and the control of such visions. They know where to fix their gaze, how to steady the scenes they want to observe, and how to see beyond the upper outward layers of the Astral Light. With the former such glimpses into the waves are hallucinations: with the latter they become the faithful reproduction of what actually has been, is, or will be, taking place. The glimpses at random caught by the medium, and his flickering visions in the deceptive light, are transformed under the guiding will of the Adept and Seer into steady pictures, the truthful representations of that which he wills to come within the focus of his perception.
Source: H.P.Blavatsky - The Theosophical Glossary