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canker sore n.
A small painful ulcer of the mucous membrane of the mouth; an aphtha. Also called aphthous stomatitis, recurrent aphthous ulcers, ulcerative stomatitis.
a small, painful ulcer of the oral cavity. Canker sores are round, shallow, white ulcers on the inner surface of the cheek or lip. They are surrounded by an inflamed area and may reach 2.5 cm (1 inch) in size. Canker sores can occur in three forms: as one to five small lesions that heal within two weeks; as relatively large ulcers exceeding 6 mm (14 inch) in diameter and persisting up to three months, leaving a scar; and as many small, superficial ulcers that appear simultaneously. The most severe cases may interfere with eating and speech.