An autapomorphy in cladistics is a derived trait that is unique to a given terminal group. That is, it is found only in one member of a clade, but not found in any others or outgrouptaxa, not even those most closely related to the group (which may be a species, family or in general any clade).
An autapomorphy is not present in the closest relative of the terminal group and also was not present in their common ancestor.
See also
Homoplasy - a trait that is found in several terminal groups but evolved independently (i.e., was not present in their common ancestor)
Synapomorphy - a trait that is found in some or all terminal groups of a clade, and inherited from a common ancestor, for which it was an autapomorphy (i.e., not present in its immediate ancestor).
Underlying synapomorphy - a synapomorphy that has been lost again in many members of the clade. If lost in all but one, it can be hard to distinguish from an autapomorphy.
