What is SOLID principle?

SOLID is a mnemonic acronym for five design principles intended to make software designs more understandable, flexible, and maintainable. These principles are: Single Responsibility Principle (a class should have only one reason to change), Open/Closed Principle (software entities should be open for extension, but closed for modification), Liskov Substitution Principle (objects of a superclass should be replaceable with objects of its subclasses without breaking the application), Interface Segregation Principle (many client-specific interfaces are better than one general-purpose interface), and Dependency Inversion Principle (depend upon abstractions, not concretions).