Literally, hidden memory. This term is used to explain the origin of experiences which people believe to be original but which are actually based on memories of events they've forgotten. For example, past life regressions such as that of Virginia Tighe's hypnotic recollections of Bridey Murphy of Cork, Ireland (Bridie Murphey Corkell), are explained as recollections of events in this life which she had forgotten.
Cryptomnesia may also explain how the apparent plagiarism of such people as Helen Keller or George Harrison of the Beatles might actually be cases of hidden memory. Harrison didn't intend to plagiarize the Chiffon's "He's So Fine" in "My Sweet Lord." Nor did Keller intend to plagiarize Margaret Canby's "The Frost Fairies" when she wrote "The Frost King." Both may simply be cases of not having a conscious memory of their experiences of the works in question.
It seems likely that most so-called past life regressions induced through hypnosis are confabulations fed by cryptomnesia.
further reading
How to Think About Weird Things, Theodore Schick, Jr. and Lewis Vaughn (Mountain View, California: Mayfield Publishing Company, 1995), ch. 3.
"Voices from Beyond: The Age-Old Mystery of Channeling," by Ted Shultz in The Fringes of Reason, ed. Ted Schultz (New York: Harmony Books, 1989).