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].