A class that cannot be instantiated (no objects can be created), existing only to allow subclasses to inherit its attributes, methods, and associations.

A class that contains one or more methods that are declared but not implemented. It has a complete interface but only a partial implementation of that interface. Concrete subclasses of an abstract class are required to flesh out the implementation by overriding the abstract methods.

A class with one or more abstract methods. C# and Java just call these “abstract methods”, but in C++ they are known as a “pure virtual method”. Abstract classes cannot be instantiated.