Transpiler
"Source-to-source compiler" — translates between two high-level languages. TypeScript → JavaScript, Sass → CSS, CoffeeScript → JS, Kotlin Multiplatform → JS.
The term is jargon; academically it's just a compiler. Babel is the JS ecosystem king — translates modern JS to older JS versions via plugins. SWC and esbuild are faster alternatives (Rust/Go). The TypeScript compiler (tsc) is primarily a typechecker; the transpilation step is "easy".