The Collaborative International Dictionary of English v.0.48:

Aphthae \Aph"th[ae]\, n. pl. [L., fr. Gr. ? (mostly in pl. ?,
   Hipp.) an eruption, thrush, fr. ? to set on fire, inflame.]
   (Med.)
   Roundish pearl-colored specks or flakes in the mouth, on the
   lips, etc., terminating in white sloughs. They are commonly
   characteristic of thrush.
   [1913 Webster]