A subclass is a class that is a specialization of another class (its superclass). Specialization or the IsA relationship means that: 1. all instances of the subclass are also instances of its superclass, 2. the intension of the subclass extends the intension of its superclass, i.e. its traits are more restrictive than that of its superclass and 3. the subclass inherits the definition of all of the properties declared for its superclass without exceptions (strict inheritance), in addition to having none, one or more properties of its own.