D2I2.
congenital

Syndactyly

Syndactyly is a condition wherein two or more digits are fused together. It occurs normally in some metatherian mammals, but is an unusual condition in humans. The term is from Ancient Greek σύν (syn) 'together' and δάκτυλος (daktulos) 'finger'.

Underlined words are explained — tap any of them.

Plain-language summary adapted from Wikipedia. Not medical advice.