A language (think programming language) for describing music. The compiler is passed in a MellowD source (*.mlod file) and outputs music in MIDI, wav, or live playback form.
You can try MellowD right in your browser without downloading anything. Visit the editor at https://spencerpark.github.io/mellowd-site/editor.html. The site for the editor is also open source for the most part if you would like to check out the source.
An interactive tutorial is in the works to make learning MellowD more enjoyable. This will be accompanied by some more concrete documentation as well.