In the classification of living organisms, a group or taxon is called monophyletic when it includes one species and all of its descendants.
Monophyly is thus opposed to paraphyly and polyphyly.
In the diagram shown below, amniotes form a monophyletic group, since sauropods and mammals have a common ancestor and represent all of the descendants of the latter.
Amniotes form a monophyletic group. © Bruno Scala/Futura-Sciences