See Canker Sore.
<gastroenterology> A type of benign mouth ulcer often caused by injury to the mucosal lining of the oral cavity, viral infection or vitamin deficiency. ... (27 Sep 1997) ... 
a small painful ulcer in the mouth, usually remaining for five to seven days and healing without scarring; seen in recurrent aphthous stomatitis.
Aphthous ulcer: A small sensitive painful ulcer crater in the lining of the mouth. Commonly called 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) have aphthous ulcers at any given time. Aphthous ulcers typically last for 10-14 days and they heal without leaving...
[n] - a blister on the mucous membranes of the lips or mouth or gastrointestinal tract
noun a blister on the mucous membranes of the lips or mouth or gastrointestinal tract