Shared characteristics that define a cladistic grouping. An apomorphy (derived or specialised character) shared by two or more groups which originated in their last common ancestor (from the Greek words apo = from and morphy = shape). Synapomorphy
A character shared by two or more taxa which was derived from their common ancestor. In cladistics, synapomorphies are used to establish phylogenies.
In cladistics, synapomorphies are used to establish phylogenies.
The synapomorphy is thus opposed to both the symplesiomorphy and the autapomorphy (homoplasy).
It is not essential to a synapomorphy that all members of a clade possess it; even if some would have secondarily (=autapomorphically for their respective lineages) lost the trait, it could still be a synapomorphy of the clade as a whole.