What is "solid"

Principle of Single Liability: A class should have only one responsibility. That is, it should only play one role.
Open/Closed Principle: Should be open for extension but closed for modification.
Principle of Liskov’s Substitution: If S is a subtype of T, then T-type objects can be replaced by S-type objects without having to change the properties of this program.
Principle of Interface Segregation: Customers should not be forced to rely on methods they do not use.
Inversion of Dependency: High-level modules should not depend on low-level modules, both should depend on abstractions; abstractions should not depend on details, details should depend on abstractions.