substratum (n.)

"horizontal layer," 1590s, from a Modern Latin special use of Latin stratum "thing spread out, coverlet, bedspread, horse-blanket; pavement," noun uses of neuter of stratus "prostrate, prone," past participle of sternere "to spread out, lay down, stretch out" (from nasalized form of PIE root *stere- "to spread").

1810, "a substratum, that which is laid or spread under" in any sense, from Modern Latin substratum, noun use of neuter singular past participle of Latin substernere "to spread underneath," from sub "under, below, beneath" (see sub-) + sternere "to spread out, lay down, stretch out" (from nasalized form of PIE root *stere- "to spread"). For linguistics use, see substratum.