Engram

Engram \EN-gram\ , noun;
1. The supposed physical basis of an individual memory in the brain
2. The presumed encoding in neural tissue that provides a physical basis for the persistence of memory; a memory trace

This word dates from 1904 and comes from German engramm which is derived from Greek ἐν (en- prefix) + γράμμα ("letter").

I always find affixes to be pretty interesting because they are often multi-purpose, so their definitions can be tricky to nail down. I also like them because the ability to skillfully manipulate affixes - particularly old ones - is a true testament to language fluency.
The affix we're dealing with today is en- which is very similar in form and function to in- (and identical to em-, which is different for phonological reasons). However, en- and in- are not cut from the same cloth. English en-/em- words generally come from French, although it is a Latin prefix, so any romance language can contribute a word with this form. En- and em- turn nouns and adjectives into verbs or alter other verbs to convey a sense of putting something into something else or becomes entrenched with something. For example: tangle (noun, "twisted together, caught") → entangle (verb, "cause to become twisted together with or caught in")
In- is a native English prefix so there are a lot of archaic usages that can be applied, but generally speaking it conveys a sense of "in, within, internal." So if you are heading inland, you are moving toward the internal part of the land in relation to the sea.
Of course, it is trickier than this in actual usage because linguistic biases have made French-sounding words more and less fashionable at different points in time. As a result many in- words have been written as en-/em- words at different times, and vice versa.

Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Wednesday, December 29
Etymologies come from the Oxford English Dictionary and/or Etymonline.com