1. bodily sac for liquid or gas: an organ or other body part for storing a liquid or gas, especially the sac that stores urine urinary bladder or the sac that stores bile gallbladder
2. inflatable inner bag: an inflatable part of something, especially a football, that resembles a bag
3. botany sac in plant: a sac found in plants such as bladder wrack that stores air to help the plant to float or, as in bladderwort, traps insects
4. medicine fluid-filled blister: a blister or small sac filled with fluid
