Final answer:
The term for the lungs' tendency to return to their original size after expansion is known as 'elastic recoil,' which is critical in the passive process of normal expiration.
Step-by-step explanation:
The tendency for the lungs to return to their original size after being distended is called elastic recoil. This occurs as part of the passive process of normal expiration. When the air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior, helping to force air out upon exhalation. During this process, the diaphragm and the intercostal muscles relax, which causes the thoracic cavity to decrease in volume and increase in intrapulmonary pressure. This pressure difference between the lungs and the environment causes air to leave the lungs passively, without the need for muscle contraction.