substratum

An underlying layer.

A layer of earth beneath the surface soil; subsoil.

A foundation or groundwork.

The material on which another material is coated or fabricated.

The characterless substance that supports attributes of reality.

A substrate.

A substrate.

That which is laid or spread under; a stratum lying under another; in agriculture, the subsoil; hence, anything which underlies or supports: as, a substratum of truth.

In metaphysics, substance, or matter, as that in which qualities inhere.