A measurement in which the number of individuals in a study population displaying one or more tumors of the stomach (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) at a point in time or developing stomach tumors within a determined period of time, are compared to the total number of individuals in the study population. Often expressed as a percentage but could be expressed as a ratio.