I agree with munificent. ‘Transpiler’ implies very similar semantics between the source and target language; ‘compiler’ implies fairly different semantics (where a translation between the source and target is not obvious).
Does that make gcc a transpiler for converting portable assembly to platform-specific assembly due to their similar semantics?
GCC is an optimizing compiler, at which point all bets are off for obvious/trivial translation between C and asm. You could, hypothetically, write a optimizing TypeScript compiler that emits asm.js.
However, while you can reasonably argue C started as a portable assembly, significant differences like a type system, pointer arithmetic, a calling convention, etc, exist now. I don't think it's unreasonable to say C is significantly higher-level than assembly.
1. My parenthetical at the end was anticipating this—then again, the translation between C and asm frequently is easy, albeit tedious.
And in the other direction one could argue that those so-called "transpilers" actually compile from a higher-level language when you consider complex type-inference and static type checking. Optimizing is not all a compiler does.
First, the "semantics" had better not change, either by an optimizing or non-optimizing compiler, or you've got a miscompile.
Second, I've seen several source-to-source compilers (that the proponents of the term transpiler would certainly call transpilers) that do non-trivial transformation, e.g. to try to capture certain idioms, or that are required as a result of an "impedance mismatch" between the languages.