Short answers, pulled from the story.
SOLID is a mnemonic acronym for five principles intended to make source code more understandable, flexible, and maintainable. The five principles are the single-responsibility principle, the open-closed principle, the Liskov substitution principle, the interface segregation principle, and the dependency inversion principle.
Software engineer and instructor Robert C. Martin introduced the basic principles of SOLID design in his 2000 paper Design Principles and Design Patterns, which discussed software rot. The SOLID acronym itself was coined around 2004 by Michael Feathers.
The single-responsibility principle states that there should never be more than one reason for a class to change, meaning every class should have only one responsibility. It improves maintainability, testability, and flexibility.
The open-closed principle states that software entities should be open for extension but closed for modification. It supports extensibility, stability, and flexibility by allowing new features to be added without modifying existing code.
The Liskov substitution principle states that functions using pointers or references to base classes must be able to use pointers or references of derived classes without knowing it. It enables polymorphism, reliability, and predictability.
Although the SOLID principles apply to object-oriented programming, they also form a core philosophy for methodologies such as agile software development and adaptive software development. They apply in functional programming as well as object-oriented programming.