A substratum of something is a layer that lies under the surface of another layer, or a feature that is less obvious than other features. its deep substratum of chalk. substrata a layer that lies beneath another layer, especially in the earth
[ 's&b-"strA-t&m, -"stra ] (noun.) 1631. Medieval Latin, from Latin, neuter of substratus, past participle of substernere to spread under, from sub- + sternere to spread; more at STREW.