I think derivation as a new term of art is reasonable, because it's something that did previously not exist.
In particular it's not an "elaborate build script. The actual derivation is basically just a json file¹ with a path-name that's a function of its contents that includes specifying everything that goes into the build (env-var settings and arguments, the input sources, other required derivations, the build script, the standard environment -- all with transitively hashed paths) as well as what comes out of it.
They could maybe have called it build-spec, or something like that, but I think giving a novel thing a novel name is in fact novel is less confusing than trying to repurpose some existing terminology and trick people into thinking it's something they are already familiar with when actually it isn't.
¹ It's not actually json, but that's just some historical accident; `nix show-derivation ...` will actually output the derivation as pretty-printed json. But the key thing is that it's just simple data, not an executable script.
In particular it's not an "elaborate build script. The actual derivation is basically just a json file¹ with a path-name that's a function of its contents that includes specifying everything that goes into the build (env-var settings and arguments, the input sources, other required derivations, the build script, the standard environment -- all with transitively hashed paths) as well as what comes out of it.
They could maybe have called it build-spec, or something like that, but I think giving a novel thing a novel name is in fact novel is less confusing than trying to repurpose some existing terminology and trick people into thinking it's something they are already familiar with when actually it isn't.
¹ It's not actually json, but that's just some historical accident; `nix show-derivation ...` will actually output the derivation as pretty-printed json. But the key thing is that it's just simple data, not an executable script.