aphthous stomatitis
Medical term for painful, recurring mouth ulcers (canker sores).
Aphthous stomatitis is a condition in which small white ulcers form in the mouth.
General term for stomatitis (mouth inflammation); アフタ性口内炎 specifically refers to the recurrent aphthous type.
From アフタ (aphtha, from Greek) + 性 (-sei, 'nature') + 口内炎 (kōnaien, 'stomatitis').