A small sensitive painful ulcer crater in the lining of the mouth. Commonly called a canker sore.Aphthous ulcer: A small sensitive painful ulcer crater in the lining of the mouth. Commonly called a canker sore. Picture of a canker sore Aphthous ulcers are one of the most common problems that occur in the mouth. About 20% of the population (1 in 5 people) haveaphthous ulcers at any given time. Aphthous ulcers typically last for 10-14 days and they heal without leaving a scar. There are many possible causes ofaphthous ulcers. Often the cause is unknown. For a full article, see Canker sores (Aphthous ulcers).