synapomorphy (syn¦apo|morphy)

noun (plural synapomorphies)

• the possession by two organisms of a characteristic (not necessarily the same in each) that is derived from one characteristic in an organism from which they both evolved: synapomorphy delineates the relative status of particular features with respect to a specific evolutionary problem
• [count noun] a characteristic derived in this way: the study of skulls of this primitive reptile has led to the discovery of numerous synapomorphies with turtles