Liskov Substitution principle (LSP) states that,
Methods that use references to the base classes must be able to use the objects of the derived classes without knowing it
This principle was written by Barbara Liskov in 1988.
The idea here is that the subtypes must be replaceable for the super type references without affecting the program execution.
This principle is very closely related to Open Closed Principle(OCP), violation of LSP in turn violates the OCP. Let me explain: