A set of classes and their interrelationships.
One class may be a specialisation (a "subclass" or "derived class") of another which is one of its "superclasses" or "base classes".
When a method is invoked on an object it is first looked for in the object's class, then the superclass of that class, and so on up the hierarchy until it is found. Thus a class need only define those methods which are specific to it, and inherits methods from all its superclasses.
See also: multiple inheritance.