Gastric cancer is a disease in which malignant (cancer) cells form in the lining of the stomach. The stomach is a J-shaped organ in the upper abdomen.It is part of the digestive system, which processes nutrients (vitamins, minerals, carbohydrates, fats, proteins, and water) in foods that are eaten and helps pass waste material out of the body.
https://www.cancer.gov/types/stomach/patient/stomach-treatment-pdq

