This is a term describing the seeing of images when someone is not hallucinating.
The general term for such events is 'pareidolia' - a psychological occurrence in which we look at a vague, essentially random stimulus, a sound or image, and decide that it is something very specific and significant. Harmless examples including seeing images and faces in the clouds or in flames, or the "man in the moon". Usually enjoyable, innocuous, temporary, and we know exactly what we are doing.
What we see within a complex, vague and essentially meaningless image can be revealing of what we have on our mind, and what we may be expecting - that is the basis of the Rorschach Test, once popular but now rather little used. In the Rorschach, you'd be shown a series of ink-blots, and asked to describe what they look like to you. You might see a bunch of flowers. Someone else facing the same blot, might see a flight of man-eating bats. There's no "right answer" to such tests, but how one interprets their ambiguity can be revealing.
More troublesome examples occasionally give rise to urban myths and legends. For instance there is the occasionally recurring concern that sinister hidden, even Satanic, messages are hidden within popular recordings, and revealed only when these are played backwards. Led Zeppelin and Queen, among others, were accused of "back-masking" Satanic and/or pro-drug content in their recordings. In ridiculous public furores, some American states made fools of themselves passing laws about this and even requiring warning labels for this non-existent threat. As CDs replaced LPs, and backward playing of music became difficult (and before digital editing made it easy again!), the fuss died down.
In fact no such messages have ever been convincingly demonstrated, but playing any track backwards produces strange sounds within which, like seeing faces in the clouds, you might fancy that you can identify words. The "messages" described are inevitably very revealing of the mind of the person "discovering" them, rather than actually contained within the sounds.
Other eccentrics, such as a Dr Raudive, discovered innocent but daft "messages" within white noise amongst radio static and other such random noise. And remember the recently famous "face on Mars" where a pattern of shadows on the red planet did indeed resemble a cartoon face. And so it goes on - sensation in Japan at a "face" in a tomato; a Koranic inscription discerned in a fish.
Discovering religious imagery within ambiguous stimuli is actually quite common. Thus we have had such oddities as supposed images of Jesus within a piece of burned toast, or the Virgin Mary seen in a toasted cheese sandwich, in a fence post near a beach, or stains on a wall. Often hundreds or thousands of people travel to see these odd objects in pilgrimages, suggesting a more widespread hunger for miracles than most people would acknowledge. Otherwise, the objects are sold for surprisingly large sums on EBay ($28,000 for the Virgin piece of toast).
(Professor M.A. Simpson, aka CyberShrink, June 2007)