sac
[C] Countable
pl: sacs
This term carries a clinical, biological tone. It describes a closed, flexible container that is integrated into a living organism, evoking an image of a membrane-bound pocket rather than a rigid vessel. In medical or zoological contexts, it is used to describe specific anatomical features like the air sacs in birds or the amniotic sac in mammals. It is rarely used in casual conversation unless referring to specific biological phenomena.
Countable when referring to individual biological pouches, such as a venom sac or a seed sac.