In Dianetics and Scientology, an engram is defined as "a mental image picture which is a recording of an experience containing pain, unconsciousness and a real or fancied threat to survival. It is a recording in the reactive mind of something which actually happened to an individual in the past and which contained pain and unconsciousness ... It must, by definition, have impact or injury as part of its content. These engrams are a complete recording, down to the last accurate detail, of every perception present in a moment of partial or full unconsciousness."
The term engram was coined in 1904 by the German scholar Richard Semon, who defined it as a "stimulus impression" which could be reactivated by the recurrence of "the energetic conditions which ruled at the generation of the engram."
Semon's concept was re-used by L. Ron Hubbard when he published Dianetics: The Modern Science of Mental Health in 1950. He conceived of the engram as a form of "memory trace", an idea which had long existed in medicine. According to Dr. Joseph Winter, a physician who collaborated in the development of Dianetics, Hubbard had taken the term "engram" from the 1936 edition of Dorland's Medical Dictionary, where it was defined as "a lasting mark or trace .... In psychology it is the lasting trace left in the psyche by anything that has been experienced psychically; a latent memory picture." He had originally used the terms "Norn", "comanome" and "impediment" before alighting on "engram" following a suggestion from Winter.
Hubbard's concept of the engram evolved over time. In Dianetics: The Modern Science of Mental Health, he wrote that "The word engram, in dianetics is used in its severely accurate sense as a 'definite and permanent trace left by a stimulus on the protoplasm of a tissue'", which followed fairly closely the original definition in Dorland's. He later repudiated the idea that an engram was a physical cellular trace, redefining his concept as being "a mental image picture of a moment of pain and unconsciousness".
According to Hubbard whenever an engram is stimulated it increases in power.
Jeff Jacobsen has drawn an analogy between Dianetics auditing and abreaction therapy, equating engrams to the painful subconscious memories that abreaction therapy brings up to the conscious mind. He quotes Dr. Nathaniel Thornton, who compares abreaction therapy in turn to confession.