The internal name (also used by many in the Creatures Community) of the game engine used in Creatures Adventures, Creatures 3, Docking Station and Seamonkeys, among other games.
It is based around the concept of agents and indeed the entire in-game GUI is written as agents, unlike previous versions of Creatures which used MFC to provide a normal Windows GUI.
A complete rewrite of the original Creatures source code, the engine is written in standards-compliant C++ and runs under both Windows and Linux, although the Windows version is superior in a few ways, especially in the area of audio.
It's main file format is the PRAY format, although it also uses the Creatures Archive format (essentially a basic serialization scheme), plus various media formats - such as S16/C16 files (sprites/animations) and WAV/MNG files (audio and music respectively).
Under Windows, a complicated scheme involving shared memory and mutexes is required for third-party programs to communicate with the engine, but under Linux, you can communicate with the engine using CAOS using a locally bound TCP/IP port, the number of which is stored in ~/.creaturesengine/port while the engine is running.
Also known as the CEE. A derivative of this is the c2e.