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
Open-closed principle:
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.