The Collaborative International Dictionary of English v.0.48:

Heartburn \Heart"burn`\ (-b[^u]rn`), 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 other complaints.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heartburn
    n 1: 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,
         pyrosis]