A vesicle is a tiny cavity or sac in an animal, even a human animal. Vesicles are like bladders or blisters, and they usually hold fluid. They’re really small.
Vesicle is from the Latin word vesicular for “bladder or blister.” A vesicle is like a little bladder, because it's a fluid-filled sac in an animal, like little organic bags of liquid. The other key element of a vesicle is that it's small — vesicles tend to be extremely tiny. They’re so small, that some are inside cells, as membrane-bound spheres that are separate from the surrounding cytoplasm. That’s tiny!