Wikipedia
WebAssembly or wasm is an experimental efficient low-level programming language for in- browser client-side scripting, which is currently in development. Its initial aim is to support compilation from C and C++, though other source languages are also intended to be supported. WebAssembly is a portable abstract syntax tree which is designed to be faster to parse than JavaScript, as well as faster to execute. The initial implementation of WebAssembly support in browsers will be based on asm.js and PNaCl. After the minimum viable product (MVP) release, there are plans to support garbage collection which would make WebAssembly a compilation target for garbage collected programming languages like Java and C#. The team working on WebAssembly includes people from Mozilla, Microsoft, Google and Apple.
WebAssembly was first announced on 17 June 2015 and on 15 March 2016 was demonstrated executing Unity's Angry Bots in Firefox, Chromium, Google Chrome, and Microsoft Edge.