In common cladistic usage, a monophyletic group is a taxon (group of organisms) which forms a clade, meaning that it consists of an ancestral species and all its descendants. The term is synonymous with the uncommon term holophyly. Monophyletic groups are typically characterized by shared derived characteristics (synapomorphies).
Monophyly is contrasted with paraphyly and polyphyly, as shown in the second diagram. A paraphyletic group consists of all of the descendants of a common ancestor minus one or more monophyletic groups. Thus, a paraphyletic group is 'nearly' monophyletic (hence the prefix 'para', meaning 'near' or 'alongside'.) A polyphyletic group is characterized by convergent features or habits (for example, night-active primates, fruit trees, aquatic insects); the features by which the group is differentiated from others are not inherited from a common ancestor.
These definitions have taken some time to be accepted. When the cladistic school of thought became mainstream in the 1960s, several alternative definitions were in use. Indeed, taxonomists sometimes used terms without defining them, leading to confusion in the early literature, a confusion which persists.
On the broadest scale, definitions fall into two groups.