Definition
WebAssembly (Wasm) is a binary instruction format for a stack-based virtual machine that executes in sandboxed environments such as browsers at near-native speed.
How it works
Languages like C, C++ and Rust are first compiled into .wasm binary modules, which the host runtime instantiates and executes; a module runs inside a sandbox, can only reach interfaces the host explicitly provides, and interoperates with JavaScript.
Usage
Games, audio/video editing and simulations on the web, portable plugin systems for desktop and server software, and lightweight server-side and edge runtimes.
Related and confusable terms
Wasm is not an operating system: an OS is the full system-software layer managing hardware, while Wasm is a sandboxed, portable execution layer on top of it. Unlike a virtual machine, it works at the process level and does not virtualize a whole machine.