A feeling of burning, warmth, heat, or pain that often starts in the upper abdomen just beneath the lower breastbone (sternum). This discomfort may spread in waves upward into your throat, and you may have a sour taste in your mouth. Heartburn is sometimes called indigestion, acid regurgitation, sour stomach, or pyrosis. It is not caused by problems with your heart, although sometimes heart problems can feel like heartburn.