Genus–differentia definition

A genus–differentia definition is a type of intensional definition, and it is composed of two parts:

For example, consider these two definitions:

Those definitions can be expressed as one genus and two differentiae:

The use of a genus (Greek: genos) and a differentia (Greek: diaphora) in constructing a definition goes back at least as far as Aristotle (384–322 BCE). Furthermore, a genus may fulfill certain characteristics (described below) that qualify it to be referred to as a species, a term derived from the Greek word eidos, which means "form" in Plato's dialogues but should be taken to mean "species" in Aristotle's corpus.

The process of producing new definitions by extending existing definitions is commonly known as differentiation (and also as derivation). The reverse process, by which just part of an existing definition is used itself as a new definition, is called abstraction; the new definition is called an abstraction and it is said to have been abstracted away from the existing definition.

For instance, consider the following:

A part of that definition may be singled out (using parentheses here):

and with that part, an abstraction may be formed: