canker, small sore on the inside of the mouth. A canker appears as a shallow, whitish ulcer surrounded by a thin, red area. It is tender, sometimes painful, and may occur singly or as one of a group of sores. Cankers develop on the inner surfaces of the lips or cheeks, on the gums, under the tongue, or on the roof of the mouth. The cause is unknown, but cankers have been associated with friction, injury, allergy, and viral infection. They generally heal by themselves in a few days but can be recurrent.