That attribute of a species that distinguishes it from other species contained in a genus. E.g., “rational” is the specific difference possessed by human beings that distinguishes them from other species contained in the genus “animal.”  Although classical logic generally reserves the term “specific difference” for the difference constituting the “metaphysical definition” of something, there are other kinds of differences that are used for definitions as well.  Properties, causes, and accidents are often used in place of a pure specific difference.  In particular, the use of the per se causes of a thing (giving rise to what was classically referred to as a “physical definition”) manifests more fully the essence under consideration.  When a definition is formed by the use of properties and, especially, accidents, such a definition is often referred to as being “descriptive.”