In the Three Domain taxonomy of life, Genus is the seventh of the eight ranks:

  1. Domain
  2. Kingdom
  3. Phylum
  4. Class
  5. Order
  6. Family
  7. Genus
  8. Species

For Homo Sapiens (us), the genus is Homo:


The genus of a surface is a topological invariant that is (roughly) the number of holes in the surface, usually denoted as g. For example, the genus of a coffee cup or a torus is 1, whilst the genus of a sphere is 0.

It is related to the Euler characteristic by the formula χ = 2 - 2g.

Strictly speaking, the genus is the number of non-intersecting closed curves that can be embedded in a surface without dividing it up.

Genus is a principal taxonomic category in organizing biological classification. It represents a collection of related species; a collection of related genera are grouped into a family. The rules surrounding the application and usage of taxonomic labels is highly formalized, and often confusing; below are some notes that I have collected for my own use when noding Earthly flora and fauna.

In modern convention, the genus name is always capitalized, while the species name never is; both are always either italicized or underlined; italicization is more common. A proper form should look like this: Genus species.

While technically only the second part of the Latin name is species specific, a species should always be referred to by both genus and species name; this is called the full specific name.

Every genus name is unique; this is not true of species names (hence we have Phaseolus vulgaris, Beta vulgaris, and Sciurus vulgaris, among dozens of other vulgarrises, but only one group with the name Phaseolus).

Taxonomic levels higher than genus are not italicized, but are capitalized.

The plural of genus is genera.

Brevity Quest 2016

Ge"nus (?), n.; pl. Genera (#). [L., birth, race, kind, sort; akin to Gr. . See Gender, and cf. Benign.]

1. Logic

A class of objects divided into several subordinate species; a class more extensive than a species; a precisely defined and exactly divided class; one of the five predicable conceptions, or sorts of terms.

2. Biol.

An assemblage of species, having so many fundamental points of structure in common, that in the judgment of competent scientists, they may receive a common substantive name. A genus is not necessarily the lowest definable group of species, for it may often be divided into several subgenera. In proportion as its definition is exact, it is natural genus; if its definition can not be made clear, it is more or less an artificial genus.

⇒ Thus in the animal kingdom the lion, leopard, tiger, cat, and panther are species of the Cat kind or genus, while in the vegetable kingdom all the species of oak form a single genus. Some genera are represented by a multitude of species, as Solanum (Nightshade) and Carex (Sedge), others by few, and some by only one known species.

Subaltern genus Logic, a genus which may be a species of a higher genus, as the genus denoted by quadruped, which is also a species of mammal. -- Summum genus [L.] Logic, the highest genus; a genus which can not be classed as a species, as being .


© Webster 1913.

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