A group of organisms that includes their most recent common ancestor and some but not all of its descendents. (cf. monophyletic; technically speaking, the distinction between polyphyletic and paraphyletic is not as clear as it might seem.) A taxon in which either the ancestral species or some descendent species have been removed by systematists to another group, usually because of morphologic distinctiveness.
For example: "fish" is a paraphyletic group because it does not include the other vertebrates, which undoubtedly also derive from the ancestral fish species.