Source : Free On-Line Dictionary of Computing

Liskov substitution principle
     
         (LSP) The principle that
        {object-oriented} {functions} that use {pointers} or
        references to a {base class} must be able to use {objects} of
        a {derived class} without knowing it.
     
        Barbara Liskov first wrote it as follows: If for each object
        o1 of type S there is an object o2 of type T such that for all
        programs P defined in terms of T, the behaviour of P is
        unchanged when o1 is substituted for o2 then S is a {subtype}
        of T.
     
        A function that violates the LSP uses a reference to a base
        class and must know about all the derivatives of that base
        class.  Such a function violates the {open/closed principle}
        because it must be modified whenever a new derivative of the
        base class is created.
     
        [Liskov, B. Data Abstraction and Hierarchy, SIGPLAN
        Notices. 23(5), May 1988].
     
        (2001-09-14)