WebAssembly (MDN Web Docs)

WebAssembly is a new type of code that can be run in modern web browsers — it is a low-level assembly-like language with a compact binary format that runs with near-native performance and provides languages such as C/C++ and Rust with a compilation target so that they can run on the web. WebAssembly has huge implications for the web platform — it provides a way to run code written in multiple languages on the web at near native speed, with client apps running on the web that previously couldn’t have done so. This allows you to take advantage of WebAssembly’s performance and power and JavaScript’s expressiveness and flexibility in the same apps, even if you don’t know how to write WebAssembly code.

Source: developer.mozilla.org