1. A fluid-filled space that is either within or beneath the epidermis (outer skin): "In young adults, lung bullae are usually congenital defects (abnormalities present from birth)."

"In later life, bullae develop in patients with the over distention (stretching) and coalescence (growing together) of the alveoli (tiny air sacs) in the lungs."

2. A large blister or skin vesicle filled with fluid: "A bulla in the skin is simply a large blister."

"Blisters on the skin are called bullae while bullae on the pleura (the membrane covering the lung) are also called blebs."

3. Etymology: from Medieval Latin which came from Latin, "bubble, seal". It referred to any rounded protrusion, particularly one that was hollow or cystic (an abnormal sac in the body, filled with a fluid or semisolid and enclosed in a membrane).

The bony projection of the temporal bone containing the tympanic cavity in many mammals.