KallistiOS (KOS) is a free, BSD licensed development library for the Sega Dreamcast game console created by a team of free software developers over the internet. Development on this development library began in 2000, under the name of libdream. In late 2000, libdream changed focus from a development library to a system to run embedded Dreamcast games simultaneously, similar to embedded operating systems. At this time, libdream was changed to KallistiOS to more accurately reflect the change of scope of the project. Some time later, the scope was changed once again back to the original intention of being a development library, and away from the idea of an embedded OS. Regardless of the fact that KallistiOS is no longer an OS at all, it was decided that the name would not be changed. KallistiOS has been used to create both freely available freeware independent games and commercial independent games such as Cryptic Allusion's Feet of Fury, S+F Software's Inhabitants, JMD's Maqiupai, and HarmlessLion's Cool Herders.
Kallisti, meaning "to the fairest" in Greek, was the inspiration for naming the system. Pronunciations include Kallisti-O's (like a breakfast cereal); Kallisti O.S. (like an operating system); or the short abbreviation, KOS, may be pronounced as "chaos".