What are the SOLID principles in software engineering?

SOLID is five OO design principles that improve maintainability: - **S (SRP):** One reason to change. - **O (OCP):** Open for extension, closed for modification. - **L (LSP):** Subtypes must be substitutable for base types. - **I (ISP):** Many small interfaces, not one giant interface. - **D (DIP):** Depend on abstractions, not concretions. **Interview tip:** Give a practical example (e.g., swapping a payment provider using an interface) to show you can apply SOLID, not just recite it.