Is it ensure, insure, or assure?
"horizontal layer," 1590s, from Modern Latin stratum, special use of Latin stratum "thing spread out, coverlet, pavement," from neuter 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.)).
stratum stra·tum (strā'təm, strāt'əm)
n. pl. stra·tums or stra·ta (-tə)
A horizontal layer of material, especially one of several parallel layers arranged one on top of another.
Any of the layers of differentiated tissue forming an anatomical structure.