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.

A subclass can have more than one immediate superclass and consequently inherits the properties of all of its superclasses (multiple inheritance). The IsA relationship or specialization between two or more classes gives rise to a structure known as a class hierarchy. The IsA relationship is transitive and may not be cyclic. In some contexts (e.g. the programming language C++) the term derived class is used synonymously with subclass.
For example:
Every Person IsA Biological Object, or Person is a subclass of Biological Object.
Also, every Person IsA Actor. A Person may die. However other kinds of Actors, such as companies, don’t die (c.f. 2).
Every Biological Object IsA Physical Object. A Physical Object can be moved. Hence a Person can be moved also (c.f. 3).