polydactyly n birth defect characterized by the presence of more than the normal number of fingers or toes Usage(s) This was significant because such malformations as polydactyly (extra fingers) often occur on only one hand, sometimes skip a generation. Hydrocephalus, or water on the brain, and polydactyly, the presence of extra fingers or toes, also result from faulty genes. Synonym(s) hyperdactyly