What Is Scrying?
Scrying is a form of divination, attempting to know the unknown. The word comes from the old word 'descry' which meant to see or to discover. This in turn is believed to have originated from old French. So the word scrying means divination through visions - literal clairvoyance.
Originally the word was used specifically to refer to only to scrying via some form of reflective surface on which the visions could literally be projected. So divinatory techniques such as crystal ball gazing and mirror scrying were prime examples.
Today the word scrying tends to be used more widely to refer to almost any form of divination. This is in keeping with the original meaning of the word if we allow that 'seeing' need not be taken literally.
How Does Scrying Work?
Does scrying actually work, and if so how? The earlier use of the word came from a belief that images were physically generated on the scrying medium in a way that would be visible to any observer. Today people are more inclined to believe that the visions are in the mind of the scrier and are projected by them onto the medium; such visions would probably not be visible to any third party.
So if scrying works, as many people believe, then it's probably a case of the medium acting to help the psychic focus their own powers and imagination.