A class can be seen as a prototype for a particular kind of object. A class definition declares instance variables and defines methods for all members of the class. Objects that have the same types of instance variables and have access to the same methods belong to the same class.
Any class that's one step below another class in the inheritance hierarchy.
Occasionally the more general interpretation of subclass would be: any class that inherits from another class.
Sometimes to subclass is used to describe the process of defining a subclass of another class.