Species
Etymology
From Latin "species" (appearance, form, kind), from "specere" (to look at) — literally what a thing looks like. Linnaeus made it the fundamental rank of his classification in the 1700s.
noun · /SPEE-sheez/
A group of organisms similar enough to interbreed and produce fertile offspring, treated as one kind.
What it names
The basic unit of biological classification, the level at which we count life’s variety and mark its losses — “an endangered species.”
Example
A horse and a donkey are separate species: they can breed, but their offspring, the mule, is almost always sterile.
Related idea
A species is a single kind; biodiversity is the abundance and variety of many species together. Losing one species chips away at the biodiversity of the whole ecosystem.