SOLID is a mnemonic acronym for five object-oriented design (OOD) principles that help to create more understandable, flexible, and maintainable code. The principles are:

• Single responsibility principle (SRP): A class should have only one responsibility. This means that it should only do one thing and do it well.
• Open–closed principle (OCP): Classes should be available for extension but closed for modification. This means you should be able to add new features to a class without changing the existing code.
• Liskov substitution principle (LSP): Derived classes should be substitutable for their base classes. This means you should be able to use a derived class object instead of an object of its base class without problems.
• Interface segregation principle (ISP): Clients should not be forced to depend on interfaces they do not use. This means that you should not cause a client to implement an interface if it does not need to use all of the methods in the interface.
• Dependency inversion principle (DIP): Depend upon abstractions, [not] concretions. This means you should depend on interfaces or abstract classes rather than concrete ones. This makes your code more flexible and easier to maintain.

The SOLID principles are often used together to create well-designed and maintainable code. They can help improve your code’s readability, testability, and scalability.