Source : Free On-Line Dictionary of Computing

open/closed principle
     
         A principle used in {OOPL} which states
        that a {class} must be open and closed where open means it has
        the ability to be extended and closed means it cannot be
        modified other than by extension.
     
        The idea is that once a class has been approved for use having
        gone through code reviews, unit tests, and other qualifying
        procedures, you don't want to change the class very much, just
        extend it.  In practice the open/closed principle simply means
        making good use of {abstraction} and {polymorphism}.
     
        (1997-09-23)