stratum (n.) Look up stratum at Dictionary.com
"horizontal layer," 1590s, from 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 PIE *stre-to- "to stretch, extend," from root *stere- "to spread, extend, stretch out" (see structure (n.)).