a advanced defining feature
In cladistics, a synapomorphy or synapomorphic character state is a trait that is shared (`symmorphy`) by two or more taxa and inferred to have been present in their most recent common ancestor, whose own ancestor in turn is inferred to not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in questi.....
Synapomorphy meaning shared form) is a derived (new) character shared by groups. A synapomorphy can be used to infer common ancestry. 
a morphologic character derived in order to infer common ancestry among taxa under consideration.
Type: Term Pronunciation: si-nap′ō-mōr-fē Definitions: 1. Species-specific unique embryologic features.
A character which is derived, and because it is shared by the taxa under consideration, is used to infer common ancestry.
a derived trait that is shared by two or more taxa of shared ancestry