Roux-en-Y gastric bypass: A surgical procedure which may be done for severe obesity. The procedure involves cutting the stomach in two to create a pouch out of the smaller proximal (near) portion of the stomach, attaching it to the small intestine, bypassing a large part of the stomach and all of the duodenum. The procedure may help with weight loss because the stomach pouch is too small to hold much food and skipping the duodenum reduces the absorption of fat which is rich in calories. The procedure may be done by laparoscopy. Also called a Roux-en-Y anastomosis or Roux-en-Y gastrojejunostomy.