Polydactyly is a polymorphism in which an individual is born with an extra digit on the hand or foot. This is commonly observed in cats, especially in those cats occupying Ernest Hemingway's house in Key West. Polydactyl is both an adjective and a noun meaing polydactyly.
When a human is polydactyl, it is classified as a specific case within a genus. Polydactyl is the most common congenital hand disability.
There are a number of famous cases of polydactyl people, among them the Biblical giant Goliath, Antonio Alfonesca, and Anne Boleyn. In the story of the The Princess Bride, Inigo Montoya vows revenge against the six-fingered man who murdered his father.
All pandas appear to be polydactyl but as The Alchemist states above, their extra digit does not count.
The condition may appear in one or both of the upper or lower phalanges. The extra digit may jut off from one of the outer digits or from within the center. In the case of the latter there is a greater chance of functionality. If the digit is deemed non-functional it may be removed by a plastic or orthopedic surgeon, most easily upon birth.
Polydactyly can occur even if there is no family history of the condition. It may skip a generation and it may not.