An abdomen (also gut, belly, tummy, midriff, tucky, bingy, breadbasket, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal cavity. In arthropods, it is the posterior tagma of the body; it follows the thorax or cephalothorax.

In humans, the abdomen stretches from the thorax at the thoracic diaphragm to the pelvis at the pelvic brim. The pelvic brim stretches from the lumbosacral joint (the intervertebral disc between L5 and S1) to the pubic symphysis and is the edge of the pelvic inlet. The space above this inlet and under the thoracic diaphragm is termed the abdominal cavity. The boundary of the abdominal cavity is the abdominal wall in the front and the peritoneal surface at the rear.

In vertebrates, the abdomen is a large body cavity enclosed by the abdominal muscles, at the front and to the sides, and by part of the vertebral column at the back. Lower ribs can also enclose ventral and lateral walls. The abdominal cavity is continuous with, and above, the pelvic cavity. It is attached to the thoracic cavity by the diaphragm. Structures such as the aorta, inferior vena cava and esophagus pass through the diaphragm. Both the abdominal and pelvic cavities are lined by a serous membrane known as the parietal peritoneum. This membrane is continuous with the visceral peritoneum lining the organs. The abdomen in vertebrates contains a number of organs belonging to, for instance, the digestive system, urinary system, and muscular system.

The abdominal cavity contains most organs of the digestive system, including the stomach, the small intestine, and the colon with its attached appendix. Other digestive organs are known as the accessory digestive organs and include the liver, its attached gallbladder, and the pancreas, and these communicate with the rest of the system via various ducts. The spleen, and organs of the urinary system including the kidneys, and adrenal glands also lie within the abdomen, along with many blood vessels including the aorta and inferior vena cava. The urinary bladder, uterus, fallopian tubes, and ovaries may be seen as either abdominal organs or as pelvic organs. Finally, the abdomen contains an extensive membrane called the peritoneum. A fold of peritoneum may completely cover certain organs, whereas it may cover only one side of organs that usually lie closer to the abdominal wall. This is called the retroperitoneum, and the kidneys and ureters are known as retroperitoneal organs.

There are three layers of muscles in the abdominal wall. They are, from the outside to the inside: external oblique, internal oblique, and transverse abdominal. The first three layers extend between the vertebral column, the lower ribs, the iliac crest and pubis of the hip. All of their fibers merge towards the midline and surround the rectus abdominis in a sheath before joining up on the opposite side at the linea alba. Strength is gained by the criss-crossing of fibers, such that the external oblique runs downward and forward, the internal oblique upward and forward, and the transverse abdominal horizontally forward.

The transverse abdominal muscle is flat and triangular, with its fibers running horizontally. It lies between the internal oblique and the underlying transverse fascia. It originates from the inguinal ligament, costal cartilages 7–12, the iliac crest and thoracolumbar fascia. Inserts into the conjoint tendon, xiphoid process, linea alba and the pubic crest.

The rectus abdominis muscles are long and flat. The muscle is crossed by three fibrous bands called the tendinous intersections. The rectus abdominis is enclosed in a thick sheath, formed as described above, by fibers from each of the three muscles of the lateral abdominal wall. They originate at the pubis bone, run up the abdomen on either side of the linea alba, and insert into the cartilages of the fifth, sixth, and seventh ribs. In the region of the groin, the inguinal canal, is a passage through the layers. This gap is where the testes can drop through the wall and where the fibrous cord from the uterus in the female runs. This is also where weakness can form, and cause inguinal hernias.

The pyramidalis muscle is small and triangular. It is located in the lower abdomen in front of the rectus abdominis. It originates at the pubic bone and is inserted into the linea alba halfway up to the navel.