From the Ancient Greek word “branch”, a clade is exactly that, a branch in the “tree of life”. It is a monophyletic group, which is to say a group of organisms consisting of the single common ancestor and all of its lineal descendants. Any or all of the member species can be extant or extinct.
An incomplete clade is termed a paraphyletic group. Two clades may be sisters if they share an immediate common ancestor.
The term “clade” was coined in 1957 by Julian Huxley when referring to the product of cladogenesis which is a concept first stated by Bernhard Rensch and describes the evolutionary splitting of a parent species into two distinct species.