English▸ noun a hollow, flexible structure resembling a bag or pouch:
a fountain pen with an ink sac.
▪ a cavity within an organism, enclosed by a membrane and containing air, liquid, or solid structures.
▪ the distended membrane surrounding a hernia, cyst, or tumour.
– ORIGIN mid 18th century (as a term in biology): from French sac or Latin saccus ‘sack, bag’.