Atrioventricular septal defect (AVSD) is a congenital heart condition characterized by a defect in the septum, the wall that separates the heart's upper and lower chambers. This defect results in abnormal blood flow between the atria and ventricles, leading to inefficient circulation and increased workload on the heart. AVSD can range from partial to complete, depending on the extent of the septal defect and involvement of the atrioventricular valves.