A genus (plural, genera) is a category in biological classification, referring to groups of species that are closely related phylogenetically (that is, through their evolutionary history) and share many elements of their morphology, biochemistry, and behavior. The genus is the principal taxonomic category between species and family. Related genera are separated from others in the same family on the basis of differences in their taxonomic characters. Although the degree of difference is not precisely defined by biologists, it is greater than that occurring between closely related species, and less than that between families.

All species are designated by a latinized, two-word name called a binomial. That of humans, for example, is Homo sapiens. The first word of the binomial refers to the genus of the species. In the particular case of Homo, only one living species occurs in the genus, which is referred to as monospecific. Most genera are polyspecific and contain more than one species. This is the case of the plants known as sedges (Carex), and the insects known as fruit-flies (Drosophila), each of which has hundreds of species in the genus.