Wikipedia
Csound is a computer programming language for sound, also known as a sound compiler or an audio programming language, or more precisely, an audio DSL. It is called Csound because it is written in C, as opposed to some of its predecessors.
It is free software, available under the LGPL.
Csound was originally written at MIT by Barry Vercoe in 1985, based on his earlier system called Music 11, which in its turn followed the MUSIC-N model initiated by Max Mathews at the Bell Labs. Its development continued throughout the 1990s and 2000s, led by John ffitch at the University of Bath. The first documented version 5 release is version 5.01 on March 18, 2006. Many developers have contributed to it, most notably Istvan Varga, Gabriel Maldonado, Robin Whittle, Richard Karpen, Michael Gogins, Matt Ingalls, Steven Yi, Richard Boulanger, and Victor Lazzarini.
Developed over many years, it currently has nearly 1700 unit generators. One of its greatest strengths is that it is completely modular and extensible by the user. Csound is closely related to the underlying language for the Structured Audio extensions to MPEG-4, SAOL.