engram
A postulated physical or biochemical change in neural tissue that represents a memory.
I use the word engram to denote this permanent change wrought by a stimulus; the sum of such engrams in an organism may be called its "engram-store," among which must distinguish inherited from acquired engrams.
A painful, negative mental image representing a past event.
Where it comes from
Borrowed from German Engramm which was coined by German evolutionary biologist Richard Semon in 1904. First attested in English in 1921 (see quotation below). Ultimately from Ancient Greek ἐν (en, “in”) + γράμμα (grámma, “writing, picture”).
Adapted from the Wiktionary entry engram (CC BY-SA 4.0).