An engram is said to be a memory trace, one possible explanation for the persistence of memory. Fundamentally, an engram is posited to be a physical, biochemical change in the brain (and other neural tissue) in response to external stimuli, thus forming a memory.

Psychologist Karl Lashley coined the term and did extensive, yet unsuccessful research to localize engrams in the 1920s. However, Lashley only looked at the cereberal cortices as possible locations of the engram. Later researcher, Richard F. Thompson , sought the engram of memory in the cerebellum instead of the cereberal cortex .

Thompson and his colleagues used classical conditioning of the eyelid response in rabbits in their search for an engram. They puffed air upon the cornea of the eye and paired it with a tone. At first the rabbit didn't blink with the tone, but after a number of trails they conditioned the rabbits to blink when they heard the tone. During the experiment, they monitored several brain cells trying to located the engram.

One area that they monitored that they thought was the possible memory engram was the lateral interpositus nucleas (LIP), when deactivate by chemicals resulted in the rabbits, who were previously conditioned to blink when hearing the tone, to act as if the conditioning never took place! However, when they re-activated the LIP, they responded to the tone again with an eyeblink. This gives evidence that the LIP is the engram for classical conditioning. (James W. Kalat, Biological Psychology p.391-393)

Engrams are a central concept in L. Ron Hubbard's Dianetics psychoanalysis techniques and the associated religion of Scientology.