SOLID

In software engineering, SOLID is a mnemonic acronym for five design principles intended to make object-oriented designs more understandable, flexible, and maintainable. The principles are a subset of many principles promoted by American software engineer and instructor Robert C. Martin, first introduced in his 2000 paper Design Principles and Design Patterns discussing software rot.^: 2–3

The SOLID ideas are

• The Single-responsibility principle: "There should never be more than one reason for a class to change." In other words, every class should have only one responsibility.
• The Open–closed principle: "Software entities ... should be open for extension, but closed for modification."
• The Liskov substitution principle: "Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it." See also design by contract.
• The Interface segregation principle: "Clients should not be forced to depend upon interfaces that they do not use."
• The Dependency inversion principle: "Depend upon abstractions, [not] concretes."

The SOLID acronym was introduced later, around 2004, by Michael Feathers.

Although the SOLID principles apply to any object-oriented design, they can also form a core philosophy for methodologies such as agile development or adaptive software development.