a small pimple or blister (a raised spot) on the skin, usually having an inflamed base and containing pus;
e.g. A small pustule on her face has been bothering her recently.
(noun)
a small rounded swelling or raised spot, often with distinctive coloration, resembling a blister or a pimple, especially such a spot found on a plant due to a fungal infection;
e.g. This fungus causes pustules to form on the leaves of the plant.