SOLID is a set of five design principles intended to make object-oriented designs more understandable, flexible, and maintainable. The SOLID acronym stands for:

• Single Responsibility Principle (SRP): A class should have only one reason to change, meaning that a class should have only one job.

• Open/Closed Principle (OCP): Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification.

• Liskov Substitution Principle (LSP): Objects of a superclass should be replaceable with objects of a subclass without breaking the application.

• Interface Segregation Principle (ISP): Larger interfaces should be split into smaller ones.

• Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules. Both should depend on abstractions.

These principles help reduce dependencies, make designs easier to understand, maintain, and extend, and ultimately lead to systems that are more maintainable, scalable, testable, and reusable. The SOLID principles were developed by Robert C. Martin in a 2000 essay, "Design Principles and Design Patterns," although the acronym was coined later by Michael Feathers. While these principles apply to any object-oriented design, they can also form a core philosophy for methodologies such as agile development or adaptive software development.