Medical Definition of aphtha
1: a speck, flake, or blister on the mucous membranes (as in the mouth or gastrointestinal tract or on the lips) characteristic of some diseases (as thrush)
2: one of the vesicles filled with clear serous fluid that occur in the mouth, on the udder, and in the spaces between the digits of cloven-footed animals in some diseases—usually used in plural
3: a disease (as foot-and-mouth disease) characterized by aphthae
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