(cladistics) A derived trait that is unique to a particular taxon
(cladistics) A derived trait that is shared by two or more taxa of shared ancestry
An autapomorphic character.
In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to the focal taxon (which may be a species, family or in general any clade).