, pl. aphthaeAvoid the misspelling/mispronunciation aptha. (1) In the singular, a small ulcer on a mucous membrane. (2) In the plural, stomatitis characterized by intermittent episodes of painful oral ulcers of unknown etiology that are covered by gray exudate, are surrounded by an erythematous halo, and range from several millimeters to 2 cm in diameter; they are limited to oral mucous membranes that are not bound to periosteum, occur as solitary or multiple lesions, and heal spontaneously in 1-2 weeks. SYN: aphthae minor, aphthous stomatitis, canker sores, recurrent aphthous stomatitis, recurrent aphthous ulcers, recurrent ulcerative stomatitis, ulcerative stomatitis. [G. ulceration]