a part of a plant or animal that is like a bag and often contains liquid, or a similar artificially made structure

Bursa are fluid-filled sacs throughout the body whose function is to reduce friction between muscles and tendons as they move over bones.

A sac of saline solution is then inserted into the cavity.

a type of abscess (= swollen area containing a thick yellow liquid) under the skin

Sacs of pus, called abscesses, may form in the pelvis.

a covering that surrounds a tumour