Source : Webster's Revised Unabridged Dictionary (1913)

Pyrosis \Py*ro"sis\, n. [NL., fr Gr. ? a burning, an
   inflammation, fr. ? to burn, fr. ? fire.] (Med.)
   See {Water brash}, under {Brash}.

Source : WordNet®

pyrosis
     n : a painful burning sensation in the chest caused by
         gastroesophageal reflux (backflow from the stomach
         irritating the esophagus); symptomatic of an ulcer or a
         diaphragmatic hernia or other disorder [syn: {heartburn}]