What are the SOLID principles in object-oriented design?

SOLID is five principles for maintainable OOP code: Single Responsibility (class has one reason to change), Open/Closed (open for extension, closed for modification), Liskov Substitution (subtypes must be substitutable for base types), Interface Segregation (many specific interfaces over one general), Dependency Inversion (depend on abstractions, not concretions). Following SOLID leads to code that's easier to understand, test, and modify.