Apomorphy vs. Synapomorphy — What's the Difference?

An apomorphous character.

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.

An evolved character or trait unique to a particular phylogenetic group of organisms. The vertebral column is an apomorphy of vertebrates.

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

(systematics) A derived characteristic of a clade. Any feature novel to a species and its descendants.