Engram

An engram or memory engram is an impression made physically in neural tissue of the brain by any mental stimulus, thereby explaining the persistence of memory.

Shortly before 2268, Dr. Richard Daystrom developed multitronics – a method of constructing a computer that could think, and of impressing memory engrams upon its circuits. The result was the M-5 computer, developed with Daystrom's own engrams. As Daystrom was near a nervous breakdown at the time, the M-5 was (like its predecessors M-1 through M-4) "not entirely successful". While multitronics showed some promise, it was eventually superseded by isolinear circuitry. 

Memory beams were able to influence one's memory by erasing engrams in the neural tissue. James T. Kirk experienced this first hand on Amerind. 

Telepathic memory retrieval, as used by the Ullians involved their using telepathic abilities to retrieve and expand lost or faded memory engrams of an individual. 

A memory wipe is a medical procedure designed for the purpose of selectively eliminating memory engrams from the mind of an individual. 

The Banean punished murderers by forcing them to relive their victim's last moments every fourteen hours; this was done by inserting the memory engrams of the victim into their brain. 

The Quarren edit the memory engrams of humanoids they capture in order to gather workers for their homeworld of Quarra. 

The Mari would seek out memory engrams that contained the memories of violent thoughts and remove them, because having violent thoughts was a crime on their world. This procedure was known as an engramatic purge. 

Several of Data's memory engrams were damaged after he was shot by a Son'a weapon on Ba'ku. This caused the activation of his memory loss fail-safe system. Later, on board the USS Enterprise (NCC-1701-E), Geordi La Forge replaced them. When Data was reactivated, his first inquiry was pertaining to the missing engrams. 

The memory virus is a pathogenic parasite that disguised itself as a memory engram – normally a memory so traumatic that the host suppressed it into the subconscious.