Substratum

Definition:

• (n.) That which is laid or spread under; that which underlies something, as a layer of earth lying under another; specifically (Agric.), the subsoil.
• (n.) The permanent subject of qualities or cause of phenomena; substance.

Compare substratum with other words:

basis vs. substratum

layer vs. substratum

substrate vs. substratum

substratum vs. underlay