Bulla: A fluid-filled blister more than 5 mm (about 3/16 inch) in diameter with thin walls.
A bulla on the skin is a blister. A bulla on the pleura (the membrane covering the lung) is also called a bleb.
In Latin a bulla was a "bubble, stud or knob." It referred to any rounded protrusion, particularly one that was hollow or cystic.