Intrauterine development of an organ or structure that is abnormal in form, structure, or position.

A birth defect caused by a structural abnormality or a marked deviation from the average or norm; anything that is structurally unusual or irregular or contrary to a general rule; such as, a congenital defect.

There are four clinically significant medical types of anomalies: malformation, disruption, deformation, and dysplasia (abnormal in form; for example, retinal dysplasia is abnormal formation of the retina during embryonic development).