An integral part of ANSI CommonLisp (see CommonLispHyperSpec), which supposedly made Common Lisp the first standardized ObjectOriented language (in the ANSI/ISO/IEEE sense). It is based upon GenericFunctions and MultiMethods and provides MultipleInheritance and user-defined MethodCombinations. Designed by GregorKiczales and others.
The CLOS MetaObjectProtocol as described in TheArtOfTheMetaObjectProtocol is not part of the standard.
See also: HowObjectOrientedIsClos, ClosQuestions