Gastritis - Gastritis, an inflammation or irritation of the lining of the stomach, is not a single disease. Rather, gastritis is a condition that has many causes. Common to all people with gastritis is pain or discomfort in the upper part of the belly (abdomen), sometimes called dyspepsia. Gastritis can be a brief and sudden illness (acute gastritis), a longer-lasting condition (chronic gastritis), or a special condition, perhaps as part of another medical illness.