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The original was posted on /r/rust by /u/OS6aDohpegavod4 on 2023-08-11 04:20:18.
I couldn't find any formal definition of it, at least for Rust. The Book uses it to describe "not generic" types, but at some point I had just assumed that it meant "not polymorphic", since polymorphism is an abstraction and "concrete" (at least to me) very clearly means "not abstract". As in I can look at the type and know what it is (e.g. a struct, enum, u64).
If I see a generic, I don't know what it could be (it could be many things). The same goes for trait objects, though. Enums are also a kind of polymorphism, but I always thought of them as concrete since I can see it's an enum and can look at the variants.
Are trait objects concrete types? If so, then I guess concrete type literally just means "not generic"? Or is it something else?