In chemistry, a substrate refers to either a catalytic surface upon which a chemical species reacts, a compound or mixture of compounds acted on by an enzyme, or a substance, such as agar, that provides the nutrients for the metabolism of micro-organisms.  A prime example is the iron catalyst used in the Haber process, developed in 1905 by German chemist Fritz Haber.
In human chemistry, substrate refers to the catalyst surface on which human molecules react or the base on which an organism lives; also referring to a substance such as sustenance, e.g. grains, or growing medium, as in land, that provides the nutrients for the metabolism of the organism. 
● Surface chemistry
● Surface thermodynamics
1. (a) Change, R. (1998). Chemistry 6th Ed. New York: McGraw-Hill.
(b) Clark, J. (2004). The Essential Dictionary of Science, New York: Barnes & Noble.
2. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview), (definition: "Substrate", pgs. 33). Morrisville, NC: LuLu.
(b) Thims, Libb. (2008). The Human Molecule, (preview), (keyword: substrate, pgs. 51, 65, 77). Morrisville, NC: LuLu.
● Naveh, Zev. (1987). “Biocybernetic and Thermodynamics Perspectives of Landscape Functions and Land use Patterns”, Landscape Ecology, Vol. 1. pgs. 75-83.