a small sac or cyst.

Biology . a small bladderlike cavity, esp. one filled with fluid.

Pathology . a circumscribed elevation of the epidermis containing serous fluid; blister.

Geology . a small, usually spherical cavity in a rock or mineral, formed by expansion of a gas or vapor before the enclosing body solidified.

noun

bladder , blister , cavity , cell , cyst , sac , utricle