Source : Webster's Revised Unabridged Dictionary (1913)

Heartburn \Heart"burn`\, n. (Med.)
   An uneasy, burning sensation in the stomach, often attended
   with an inclination to vomit. It is sometimes idiopathic, but
   is often a symptom of often complaints.

Source : WordNet®

heartburn
     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: {pyrosis}]