Water canker : See Canker, n., 1.

Example:


Taxonomy
Canker sores, symptoms, cancer, treatment, mouth, mouth ulcer, diagnosis, bump, ulcer, sore, health care provider, mixture, MedlinePlus Medical Encyclopedia, first appear, tongue, aphthous ulcer, tooth, nonspecific symptoms.

Defination from the web:
... The easiest home remedy is a mixture of half hydrogen peroxide and half water. Use a cotton swab to apply the mixture directly to the canker sore. ...