Stomach

The stomach is a muscular, hollow, dilated part of the digestion system which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects (mid-gut), and molluscs. It is involved in the second phase of digestion, following mastication (chewing).
The stomach is located between the esophagus and the small intestine. It secretes protein-digesting enzymes and strong acids to aid in food digestion, (sent to it via oesophageal peristalsis) through smooth muscular contortions (called segmentation) before sending partially digested food (chyme) to the small intestines.

Epigastric pain

Epigastric pain is pain that is localized to the region of the upper abdomen immediately below the ribs. Often, those who experience this type of pain feel it during or right after eating or if they lie down too soon after eating. It is a common symptom of gastroesophageal reflux disease (GERD) or heartburn. It may be associated with the gastric contents moving upward into the back of the throat, causing inflammation and a burning pain.