Exceeding the usual number; thus, in anatomy, anomalies such as a sixth finger on one hand or a third nipple (in a human).
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 809 (https://doi.org/10.1515/pac-2015-1202)