The abnormal enlargement of air spaces distal to the terminal bronchioles in the lung. In the United States, the term emphysema is often used imprecisely to refer to any member of the group of illnesses known as chronic obstructive pulmonary disease, including emphysema, chronic bronchitis, and nonallergic asthma. The collective term obstructive pulmonary disease arises because many patients have elements of all three diseases. Although histological examination of lung tissue from surgical specimens or at autopsy is the only definitive diagnostic method, advances in physiological testing and radiologic techniques usually make an earlier diagnosis possible.