Wikipedia
asm.js is an intermediate programming language designed to allow computer software written in languages such as C to be run as web applications while maintaining performance characteristics considerably better than standard JavaScript, the typical language used for such applications.
asm.js consists of a strict subset of JavaScript, into which code written in statically-typed languages with manual memory management (such as C) is translated by a source-to-source compiler such as Emscripten (based on LLVM). Performance is improved by limiting language features to those amenable to ahead-of-time optimization and other performance improvements.
Mozilla Firefox was the first web browser to implement asm.js-specific optimizations, starting with Firefox 22.