What is SOLID Principle

Single responsibility principle^[6]

A class should only 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.
See example here

Open–closed principle^[7]

"Software entities ... should be open for extension, but closed for modification."

Liskov substitution principle^[8]

"Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program." See also design by contract.

Interface segregation principle^[9]

"Many client-specific interfaces are better than one general-purpose interface."^[4]

Dependency inversion principle^[10]

One should "depend upon abstractions, [not] concretions."^[4