A condition in which a person has memories or skills which can be accessed, but which were acquired subconsciously or through subtle, subliminal means. The person in question typically has no knowledge at all of where or when these memories came from. Such memories or skills may be stored for long periods of time before surfacing, which usually occurs at strange times: hypnotic trance, meditation, autohypnosis or even as a result of head trauma (imagine getting smashed on the head and suddenly finding yourself fluent in Portuguese!).
The peculiar and often eerie nature of cryptomnesia may lead those experiencing or witnessing it to the conclusion that these memories are from some supernatural agency: part of a previous life, a demonic or spirit possession or even from divine or angelic inspiration. Researchers often examine purported spirit contacts, past life memories and the like for errors which may have been published somewhere. This is pretty much iron-clad evidence that the case is cryptomnesia and not some more mystical source.
An example of cryptomnesia is the bizarre phenomenon of xenoglossy, a condition in which a person may have skills in a language which they have never studied and may even have no memory of having contact with. Another type of cryptomnesia is automatic writing, wherein a person may hand write or type a large amount of strange stuff without consciously willing to do so. The writer may have no clue where the writing comes from, and may not remember knowing anything about the content.
A very famous early recorded case of cryptomnesia occurred in 1874. An English medium named William Stanton Moses claimed to be in contact with the spirits of two recently-departed Indian boys. This information was later shown to have been picked up by Moses from a newspaper obituary.
Another interesting example of cryptomnesia happened on a television special in 1977. A woman was hypnotized and told vivid tales of a past-life experience. She recounted a story about a famous witch who was tried and acquitted in 1566, but gave the date as 1556. This date had come from a reprint that the woman had seen in the British Museum.
While some people might feel a little disappointment over the holes blown in the case for past-life regression or spirit visitation, cryptomnesia is an extremely interesting phenomenon from a a physiopsychological perspective. Psychologist Carl Jung made the claim that cryptomnesia is a normal part of the process of learning and the process by which memories are stored and consolidated in the mind.
This phenomenon is sometimes termed cryptanamnesia or source amnesia (because the source of the memory is forgotten).
Taber's Cyclopedic Medical Dictionary, 19th edition (FA Davis, Philadelphia, 1997).
Williams, Richard "Bizarre Phenomena" (Reader's Digest, Pleasantville, NY, 1992).
The Mystica online: http://www.themystica.com/mystica/articles/c/cryptomnesia.html