The Free On-line Dictionary of Computing (30 December 2018):

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)