Apomorphy
Clades are identified (or defined) by synapomorphies (from syn-, "together"). These are a group of apomorphies which they all have, but other groups do not. For example, the five digits (fingers and toes) like ours is a synapomorphy within the vertebrates. The tetrapods are the first vertebrates with such digits. Snakes and other tetrapods which do not have digits are tetrapods because other characters, such as amniotic eggs and diapsid skulls, show they descended from ancestors that which had digits.