Common Lisp Object System (CLOS)
Common Lisp Object System (CLOS) is a part of Common Lisp that allows to use object-oriented programming techniques in your programs. It defines concepts such as object, class and method and their interaction. CLOS is the most powerful object system available in any programming language, and mastering the more peculiar aspects of it may take time. Fortunately, it is not necessary to be a CLOS expert to use it. Two orthogonal concepts of CLOS are classes and generic functions.