What does the letter L mean in the SOLID principles when developing software?

The letter L in the SOLID principles stands for Liskov Substitution Principle. This principle states that objects of a subclass should be interchangeable with objects of the superclass without violating the correctness of the program. That is, if class B inherits from class A, then objects of class B should behave in such a way that they can be used in place of objects of class A without unexpected errors or changes in logic.

An example of violating the Liskov principle is a subclass that overrides a method of the superclass in a way that changes the expected behavior or introduces exceptions not anticipated in the superclass. Adhering to this principle helps create more reliable and extendable class hierarchies.