An integral part of ANSI Common Lisp (see Common Lisp Hyper Spec), which supposedly made Common Lisp the first standardized Object Oriented language (in the ANSI/ISO/IEEE sense). It is based upon Generic Functions and Multi Methods and provides Multiple Inheritance and user-defined Method Combinations. Designed by Gregor Kiczales and others.
The CLOS Meta Object Protocol as described in The Art Of The Meta Object Protocol is not part of the standard.
See also: How Object Oriented Is Clos, Clos Questions
See original on c2.com 