What is a node class in C++?

A node class is a class that,
?    relies on the base class for services and implementation,
?    provides a wider interface to the users than its base class,
?    relies primarily on virtual functions in its public interface
?    depends on all its direct and indirect base class
?    can be understood only in the context of the base class
?    can be used as base for further derivation
?    can be used to create objects.

A node class is a class that has added new services or functionality beyond the services inherited from its base class.