Genus–differentia Definition

A genus–differentia definition is a type of intensional definition which defines a species (that is, a type — not necessarily a biological category) as a subtype of a genus satisfying certain conditions (the differentia). Thus, the definiendum in such definitions is always a species (and not an individual), while the definiens consists of two parts:

1. a genus (or family): A pre-defined term that includes the species defined as a subtype.
2. the differentia: The condition that distinguishes the species from other instances of the same 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: 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.

Note that the genus-species relation is relative. One may define "dog" as a species of the genus "animal", while "puppy" is a species of the genus "dog". Thus, whether "dog" is a species or a genus depends on context.