Definition: Aphthous ulcer, also known as a canker sore, is a small sore that forms inside of the mouth. Aphthous ulcers are usually white, yellow or light gray and sometimes have a small red border surrounding them. Aphthous ulcers commonly appear inside the cheeks, lips, on or under the tongue, the roof of the mouth and on the gums.
Also Known As: Canker Sore
