substrate
Substrate is, literally, "the thing spread beneath." Latin substernere — sub- ("under") + sternere ("to spread, lay flat, strew") — has the past participle substratus, used as a noun in substratum for the layer laid down underneath. Locke borrowed the Latin directly into English philosophy in 1690 for the bare particular that bears qualities. In the early 1800s science clipped the ending and adopted substrate for the material an enzyme acts on, the silicon a circuit is etched into, the rock a colony grows across. Underneath, the PIE root *sterh3-, "to spread out," gives English strew, strata, stratum, prostrate, consternation — and (through Latin strata via, "spread road") street.
A substrate is what something happens on. The enzyme acts on its substrate. The circuit is etched into a silicon substrate. The fungal colony grows across the substrate of rotting wood. In every domain that borrows the word, the relationship it names is the same one: not the action, not the actor, but the material that makes the action possible. The thing that is not the process but holds the process up. Inside the word, Latin still tells you exactly what shape that thing is. Sub-, under. Sternere, to spread. Substratus, spread underneath. A substrate is, by construction, a flat surface someone or something laid down so that activity could happen on it. The image is a floor being prepared.
The word entered English through philosophy, not science. John Locke needed it. In Book II of his Essay Concerning Human Understanding (1690), he used substratum for the unobservable "something" that properties inhere in — the bare particular that has redness, weight, hardness, but is not itself any of those qualities. The word named a problem more than it named an object: you can strip qualities off a thing one by one and something still seems to remain beneath them. That remainder is the substratum. Locke wasn't sure it existed. He used the Latin form anyway because the metaphor was exactly right — whatever holds the qualities up has to be spread underneath them, flat and bearing, the way a floor is spread beneath the things that stand on it.
A century later science clipped the ending. By the early 1800s chemists were using substrate for the body that a reagent acts upon — the substance the chemistry happens to, distinct from the agent making it happen. Biology and microbiology took it next: the substrate is the medium an organism grows in or on (the agar for bacteria, the bark for the lichen, the soil for the seed). Then geology, then electronics, then computing. In every borrowing, the same structural metaphor: the layer beneath, the thing acted upon, the material that enables without participating. The Latin image stays intact through every discipline that picks the word up. The substrate is always passive. It receives. It doesn't do.
Underneath the Latin sits PIE *sterh3-, "to spread out," and the family it gives English is wide. Strew on the Germanic side — to scatter, to spread loosely, the Old English verb almost untouched. Straw is the same root in noun form — the dried thing spread for animals. Stratum and strata are the Latin neuter past participle — any layer that's been laid down, geological or social. Street comes through Latin strata via, "spread road" — the Romans named their paved ways by the act of laying stone. Prostrate is pro- + sternere, "thrown forward and flat." Consternation is con- + sternere, "thoroughly thrown to the ground," the disorder you feel when something knocks the floor out from under you. The root keeps giving the same image: flatness made by spreading, ground made by laying, surface that wasn't there until someone or something put it there. A substrate is never natural ground. It is always prepared ground — flat by virtue of having been spread there.
- strew — Old English strēowian, "to scatter, to spread"; the native Germanic form of the same PIE root
- straw — Old English strēaw, "that which is strewn"; the spread thing made into a noun — dried grass laid for bedding
- stratum / strata — Latin strātum, neuter past participle of sternere; "a thing laid down" — any layer, geological or social, plural strata
- street — Latin strāta via, "spread road" (short for "paved way"); the Roman road named by the act of its construction
- substratum — Latin substratum; Locke's philosophical term for the bearer of qualities — the form English uses when the philosophy is in view
- prostrate — Latin prostrātus, from pro- + sternere; "thrown forward and flat" — the body laid down in supplication
- consternation — Latin consternātiō, from con- (intensive) + sternere; "a thorough throwing to the ground" — the disorder of having your floor knocked out
- stratify — modern formation on stratum; "to make into layers" — what geology and sociology both do with the same image
- stratosphere — modern coinage (1908) from stratum + sphere; the layered region of the atmosphere
- stratagem — Greek stratēgēma, from stratós ("army, host"); the Greek branch uses the root for armies laid out across a field — same image, military application