What is "design by contract" and how does it relate to LSP?

Design by Contract (DbC), introduced by Bertrand Meyer, is a software design approach where components define formal agreements: preconditions (what the caller must guarantee before calling), postconditions (what the callee guarantees after returning), and invariants (properties always true of the object). LSP is directly expressed in DbC terms: a subclass must accept at least as weak preconditions as the parent (can be more permissive but not stricter) and must provide at least as strong postconditions (can guarantee more but not less). If the parent guarantees a sorted list is returned, the subclass must also return a sorted list. DbC makes LSP violations detectable at the specification level before any code is written. Languages like Eiffel have built-in DbC support; others use assertions and runtime checks.