vesicle

noun

1. a small, membranous cavity, sac, or cyst; specif.,
   1. Anat., Med. a small cavity or sac filled with fluid; esp., a small, round elevation of the skin containing a serous fluid; blister
   2. Bot. a small, bladderlike sac filled with air or liquid
2. Geol. a small, spherical cavity in volcanic rock, produced by bubbles of air or gas in the molten rock