SOLID refers to five fundamental principles used in software design.The SOLID principles are Single Responsibility Principle (SRP), Open/Closed Principle (OCP), Liskov Substitution Principle (LSP), Interface Segregation Principle (ISP), and Dependency Inversion Principle (DIP).These principles ensure that software is more flexible, maintainable, and testable.By following the SOLID principles, code quality in Java can be significantly improved.