The substrate is a stratum which is underlying to another and on which it is able to exercise some sort of influence. The notion of stratum, on the other hand, means a layer or a level of something, or all of the elements that integrates with other previous to the formation of an entity.
For ecology, the substrate is the part of the biotope (area of uniform environmental conditions) where some beings living develop their vital functions and relate.
In biology, the concept of substrate has to do with the surface where an animal or a plant, which is composed of biotic factors of abiotic factors as well.
The substrate can also be a chemical species that is considered to be object of the action of one or more reagents. A compound transformed by the action of a catalyst is a substrate.
Biochemistry, a substrate is a molecule that acts on an enzyme. In other words, the enzymes have to role of catalyzing chemical reactions involved in a substrate. The union between the enzyme and the substrate forms a complex.
Then, the linguistic substrate is grammatical, lexical and phonetic influences (so-called "borrowings") that a language exerts on another. These influences are common when a people conquered or invaded by another and the language spoken originally on this territory has influence on the language which eventually replace it. The opposite phenomenon is the superstratum. In the case of the French language, it must some of its phonetic changes (h sucked, for example) to the superstratum Frankish, following the invasion of the Franks in Gaul.