DreamScript is a scripting language for the Sega Dreamcast by Paul Willarson, better known as scherzo. It is heavily based upon javascript. As such, it is mostly compatible with javascript code, except that it adds Dreamcast-specific commands for controlling Dreamcast peripherals.
An official release of DreamScript has not yet been released, but the public release of scherzo's continuation of NesterDC (NesterDC SE) uses DreamScript to power it's menu system. DreamScript is a seperate binary, so the binary could be used for DreamScript-specific games. The scripts included with DreamScript contain most of the underlying commands of DreamScript, so in essence, NesterDC SE contains everything that one would need to create their own games in DreamScript.
Scherzo has stated that a full release of DreamScript with detailed documentation will happen sometime in the future.