something that is spread or laid under something else; a stratum or layer lying under another.

something that underlies or serves as a basis or foundation.

Agriculture . the subsoil.

Biology . the base or material on which a nonmotile organism lives or grows.

Philosophy . substance, considered as that which supports accidents or attributes.

Photography . a layer of material placed directly on a film or plate as a foundation for the sensitive emulsion.

Historical Linguistics . a set of features of a language traceable to the influence of an earlier language that it has replaced, esp. among a subjugated population

The French word for 80, quatre-vingts four twenties may reflect a Celtic substratum.

noun

basis , bed , bottom , foot , footing , foundation , fundament , ground , groundwork , seat , underpinning