In object oriented programming, SOLID is for five design principles inented to make software designs more understandable, flexible and maintainable.
Single Responsibility Principle:
A class should have a single responsibility, that is, only changes to one part of the software's specification should be able to affect the specification of the class
Sotware entities should be open for extension, but closed for modification
Liskov Subsitution principle:
Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
Interface segregation principle:
Many client-specific interfaces are better than one general-purpose interface.
Dependency Inversion principle:
One should depend upon abstractions, not concretions.