Measurement of the distance a stomach tumor has penetrated into or through the adjoining tissue. A tumor is an abnormal growth of tissue resulting from uncontrolled, progressive multiplication of cells and serving no physiological function. The stomach is the curved, muscular, saclike structure that is an enlargement of the alimentary canal between the esophagus and the small intestine, and that temporarily holds food and secretes proteases and strong acids to aid in food digestion.