Transpiler is a recently made up word, and describes something that's just as well described as a compiler. I wouldn't spend too much time hoping for a distinction here.
I'd thought a transpiler is a compiler where the source and target language are the same and the semantics of the program are preserved across transpilation (eg. Closure Compiler, Babel, LLVM optimization passes). That's a useful distinction: it implies that you can feed the output of a transpiler back into the input and at worse the result will be idempotent, and that you can mix transpiled output with raw input and the result will still be interpretable.
Evidently Wikipedia has a more broad definition of source-to-source compilation, which I'd agree is pretty meaningless. The distinction between high- and low-level languages is pretty arbitrary; you can write machine-language "source code", so in some ways all compilers are source-to-source.
