the word "substratum" is a modern latin formation, entering english in the early 17th century. it is a compound of the latin prefix "sub-" (under) and the noun "stratum" (something spread or laid down). the root verb "sternere" (to spread, lay flat) is of proto-indo-european origin, related to concepts of stretching and extension. the literal meaning of "a layer spread underneath" evolved from its physical geological sense to encompass more abstract concepts, such as an underlying foundation in linguistics, sociology, and philosophy, denoting a base upon which other things are built or exist.

Structure: sub (under) + stratum (layer) - sub: a prefix of latin origin meaning "under, below, from under." - stratum: from latin stratum, meaning "a spread, bed, cover, blanket," from the past participle of sternere "to spread out, lay down, stretch out."