From Sega Retro
KallistiOS (KOS) is a free, BSD licensed development library for the Sega Dreamcast created by a team of free software developers over the internet. Work 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 homebrew and commercial games such as Cryptic Allusion's Feet of Fury, S+F Software's Inhabitants, JMD's Maqiupai, Harmlesslion's Cool Herders, and NG:DEV.TEAM's Last Hope.
KallistiOS and Kalisto
Although there is a similarity in name, the developers of KallistiOS claim there is no relationship to the warez group Kallisto, who became notorious for being the first group to release warez in a self-bootable format for the Dreamcast, eliminating the need for a bootdisk.
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".