A Bulla

1. (medicine) A blister, vesicle, or other thin-walled cavity or lesion.
2. (archaeology) A clay envelope or hollow ball, typically with seal impressions or writing on its outside indicating its contents.
3. (historical) In ancient Rome, a kind of amulet or boss.
4. Later, a handwritten document from the papal chancellery.
5. The tympanic part of a temporal bone (having a bubble-like appearance)