Liskov's notion of a behavioral subtype defines a notion of substitutability for objects;
That is, if S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program
When this occurs, we can say that design or implementation adheres with the LSP (Liskov's Substitution Principle).
It is desirable for designs & implementations to adhere with LSP.
See also, if You wish or need, ... : Liskov substitution principle on Wikipedia, SOLID, Design by Contract.