Paraphyly

Paraphyly is a term in cladistics. It means a group which does not include all its descendents. So Sauropsida without birds is paraphyletic, since birds are definitely descended from archosaurs, probably small theropod dinosaurs (origin of birds).
Paraphyly is corrected to monophyly when the missing group(s) are included. So if birds are included in Sauropsida, the larger group is then monophyletic. 
Traditional Linnaean classification is often paraphyletic. Many common terms, like "fish", are paraphyletic, but still very useful for everyday purposes.