: a painful shallow ulcer of the mouth that has a grayish-white base surrounded by a reddish inflamed area and is of uncertain cause but is not due to the virus causing herpes simplex — compare cold sore
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: a small painful ulcer especially of the mouthespecially: 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