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.

Dyspepsia

Dyspepsia, also known as indigestion or upset stomach, is a term that describes discomfort or pain in the upper abdomen. It is not a disease. Dyspepsia is a group of symptoms which often include bloating, nausea and burping. Indigestion is usually caused by stomach acid coming into contact with the mucosa of the digestive system - the sensitive protective lining of the digestive system. Stomach acids break down the mucosa, causing irritation and inflammation, which trigger the symptoms of indigestion.