substratum

noun3

1. •
   A layer that lies underneath another.

   • ..; in this case it became necessary to employ a machine like the lapidary's wheel, partially to grind away the colored surface till the white substratum appeared. — 1847
2. •
   A substrate.
3. •
   (figurative) The underlying cause or basis of something.

   • However exaggerated an estimate this might be, the substratum of truth was solid and auriferous enough to dazzle the imagination. — 1914