Genus–differentia definition

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

1. a genus (or family): An existing definition that serves as a portion of the new definition; all definitions with the same genus are considered members of that genus.
2. the differentia: The portion of the definition that is not provided by the genus.

For example, consider these two definitions:

• a triangle: A plane figure that has 3 straight bounding sides.
• a quadrilateral: A plane figure that has 4 straight bounding sides.

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

1. one genus:
   • the genus for both a triangle and a quadrilateral: "A plane figure"
2. two differentiae:
   • the differentia for a triangle: "that has 3 straight bounding sides."
   • the differentia for a quadrilateral: "that has 4 straight bounding sides."

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.