SOLID is an acronym for five principles of object-oriented design that promote maintainable, scalable, and flexible software. S - Single Responsibility Principle: A class should have only one reason to change, meaning it should have a single responsibility. O - Open/Closed Principle: Classes should be open for extension but closed for modification. L - Liskov Substitution Principle: Subtypes must be substitutable for their base types without altering the correctness of the program. I - Interface Segregation Principle: Clients should not be forced to depend on interfaces they don’t use. D - Dependency Inversion Principle: High-level modules should not depend on low-level modules; both should depend on abstractions.