The term genus reached us from the ancient Greek \(\gamma \acute{\varepsilon }\nu o\varsigma\), via Latin. It is a term already used for classifications during the times of Aristotle, as illustrated by the following extract from his “Metaphysics” [5, Book 5, Chap. 1024]:

The term “genus” (or “race”) is used: (a) When there is a continuous generation of things of the same type; e.g., “as long as the human race exists” means “as long as the generation of human beings is continuous.” (b) Of anything from which things derive their being as the prime mover of them into being. Thus some are called Hellenes by race, and others Ionians, because some have Hellen and others Ion as their first ancestor. […] (c) In the sense that the plane is the “genus” of plane figures, and the solid of solids […] (d) In the sense that in formulae the first component, which is stated as part of the essence, is the genus, and the qualities are said to be its differentiae. […]

Things are called “generically different” whose immediate substrates are different and cannot be resolved one into the other or both into the same thing. E.g., form and matter are generically different, and all things which belong to different categories of being; for some of the things of which being is predicated denote the essence, others a quality, and others the various other things which have already been distinguished. For these also cannot be resolved either into each other or into any one thing.