“An object which is used to create instances, i. e., a template, description, pattern, or "blueprint" of a category or collection of very similar items. Among other things, a class describes the interface the these items will present to the outside world, i. e., the available and appropriate methods, constants, and exceptions. A class represents an abstraction of the items. A class may itself be parameterized (i. e., it actually represents a family of very closely related classes), in which case we refer to it as a parameterized class. Class is a recursive concept. Specifically, we may define classes as being composed of other classes (i. e., heterogeneous composite classes and homogeneous composite classes), in terms of itself (a recursively defined class), as inheriting characteristics from one or more other classes (i. e., the superclasses of the class), and as providing characteristics to other classes (i. e., the subclasses of the class). In some places, classes are defined as "the set of all instances of a type," and the term "type" is given the above definition for class.”

Source: Essays on object-oriented software engineering (1993), p. 328