SOLID is an acronym for five object-oriented design principles introduced by Robert C. Martin (Uncle Bob) that guide developers toward creating maintainable, scalable, and flexible software systems. These principles address dependency management and separation of concerns, helping to reduce coupling, increase cohesion, and make code easier to test, modify, and extend. While rooted in object-oriented programming, SOLID has proven relevant across paradigms—from microservices architecture to modern TypeScript applications. Understanding when and how to apply these principles pragmatically, without over-engineering, is key to writing professional-grade code that withstands change.