Substratum is a noun that refers to a foundation or basis of something. 

The Latin prefix S-U-B means ‘below,’ while the root word stratum, also Latin in origin, refers to a layer. Something below a layer could be thought of as a foundation holding things up from beneath. Our word of the day began as a geological term that referred to the foundation beneath the earth’s surface. More recently, it’s been used to refer to any kind of foundation, for example:

The substratum of our business is good customer service. Without that firm foundation, we wouldn’t be the biggest shoe store in town.