The Collaborative International Dictionary of English v.0.48:

Bulla \Bul"la\, n.; pl. Bull[ae]. [L. bulla bubble. See Bull
   an edict.]
   1. (Med.) A bleb; a vesicle, or an elevation of the cuticle,
      containing a transparent watery fluid.
      [1913 Webster]

   2. (Anat.) The ovoid prominence below the opening of the ear
      in the skulls of many animals; as, the tympanic or
      auditory bulla.
      [1913 Webster]

   3. A leaden seal for a document; esp. the round leaden seal
      attached to the papal bulls, which has on one side a
      representation of St. Peter and St. Paul, and on the other
      the name of the pope who uses it.
      [1913 Webster]

   4. (Zool.) A genus of marine shells. See Bubble shell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bulla
    n 1: (pathology) an elevation of the skin filled with serous
         fluid [syn: blister, bulla, bleb]
    2: the round leaden seal affixed to a papal bull