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,^[1][2][3] first introduced in his 2000 paper Design Principles and Design Patterns discussing software rot.^{[2][4]: 2–3}

The SOLID ideas are

• The Single-responsibility principle: "There should never be more than one reason for a class to change."^[5] In other words, every class should have only one responsibility.^[6]
• The Open–closed principle: "Software entities ... should be open for extension, but closed for modification."^[7]
• 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."^[8] See also design by contract.^[8]
• The Interface segregation principle: "Clients should not be forced to depend upon interfaces that they do not use."^[9][4]
• The Dependency inversion principle: "Depend upon abstractions, [not] concretions."^[10][4]

The SOLID acronym was introduced later, around 2004, by Michael Feathers.^[11]

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.^[3]