1. A condition of certain climbing birds, having the toes of each foot arranged in pairs, with two toes in front and two behind: After reading about zygodactylism in her biology book, Jane found out that woodpeckers, owls and most parrots have such toe arrangements.
2. This term is sometimes used to designate simple syndactyly (webbing between adjacent digits so they are more or less completely attached), as distinguished from syndactyly in which there is bony fusion between the phalanges of the digits involved and usually occurring in the hand between the third and fourth digits and in the foot between the fourth and fifth.