A hernia is the protrusion of a structure (such as an organ or fat) through the walls of its containing cavity. This is better understood when considering examples of hernias:

Abdominal wall hernias: the protrusion of fat or bowel through a defect in the abdominal wall. The defect may be at the umbilicus (umbilical hernia), around the umbilicus (periumbilical), in the groin (inguinal, femoral or obturator), on the side of the rectus muscle (spigelian) or through a pervious surgical incision (incisional).

The point at which the hernia comes out of its defect is called the “neck” of the hernia. The hernial sac is a layer of peritoneum surrounding the hernia. The peritoneum is a smooth layer of tissue that lines the whole abdominal cavity part of which is pushed through the defect around the hernia forming the sac.

If a large hernia is eventually pushed through a smaller defect, it can get stuck (incarcerate). If the neck is too tight, blood supply to the hernial contents may be compromised which results in strangulation of the tissues. This requires emergent surgical repair. The patient presents with a painful irreducible hernia which should be repaired as soon as possible.