n :  a small painful ulcer esp. of the mouth : esp  :  a painful shallow ulceration of the oral mucous membranes that has a grayish-white base surrounded by a reddish inflamed area and is characteristic of aphthous stomatitis - compare COLD SORE  .
 
 
 
Published under license with Merriam-Webster, Incorporated.  © 1997-2004.