TC39 is an acronym for Technical Committee 39, a committee within Ecma International (formerly known as the European Computer Manufacturers Association) responsible for the development of the ECMAScript programming language. It consists of representatives from various companies such as Microsoft, Google, Apple, Mozilla and Facebook. The committee works on the specification of the language, including new features, syntax, and other aspects of the language. They also work to ensure that the language remains compatible across browsers and platforms.