Liskov Substitution
In plain terms
Subtypes must be usable wherever their base type is expected.
Going deeper
The Liskov Substitution Principle states that objects of a subclass should be substitutable for their parent without breaking behavior. Subtypes must honor the contract.
Under the hood
The 'L' in SOLID; violations (e.g., a square subclassing rectangle) cause surprising bugs.
Examples
SOLID, inheritance correctness, polymorphism, substitutability, contracts.
Think of it like...
Like any certified replacement part fitting and working where the original did.