vesicle

noun

• a fluid- or air-filled cavity or sac, in particular.
• Anatomy & Zoology a small fluid-filled bladder, sac, cyst, or vacuole within the body.
• Botany an air-filled swelling in a plant, especially a seaweed.
• Geology a small cavity in volcanic rock, produced by gas bubbles in the molten lava.
• Medicine a small blister full of clear fluid.