(Pronounced "kloss" or "c-loss")

Common LISP Object System. Extension to the LISP language that enables object oriented programming in its many forms.

Now part of Common LISP syntax, it's based on the preceding CommonLoops, Symbolics "flavors" and Xerox LOOPS.

Most Common Lisp implementations support it, including the open-source CMUCL and SBCL implementations. (It most definitely is lacking from many Lisp dialects that are not quite as broad as Common Lisp, including elisp and scheme.)

For more information, see "Common Lisp Object System", chapter 28 in Common Lisp: The Language, 2nd ed, Guy L. Steele et al., and Chapter 7 ("Objects") in X3J13. Both are available free of charge as HTML (the ANSI X3J13 standard is available as "Common Lisp HyperSpec" (CLHS).)

(Sources: FOLDOC, top of my head, random web searches =)