Symplesiomorphy vs. Synapomorphy — What's the Difference?

The possession by two or more organisms or taxa of a shared primitive or ancestral character not exclusive to or characteristic of them in particular; the shared character itself.

An evolved character or trait that is shared by two or more taxonomic groups and is considered to have originated in their common ancestor. Hair is synapomorphic for marsupials, primates, and all other taxa that are grouped together in the class Mammalia.

(cladistics) An ancestral trait shared by two or more taxa.

(cladistics) A derived trait that is shared by two or more taxa of shared ancestry.