UPDATED: The definition of hernia is "the protrusion of a deep structure through a superficial weakness of defect".

Herniation has many etiologies, but in all cases a weakness of a superficial containing wall  (usually layered) or a normal or abnormal opening (defect) must be present. A true hernia usually has a deep sac or hernia sac which contains the herniated viscus or viscera. Repair of a hernia is called a hernioplasty or a herniorrhaphy.

Although with exceptions, a herniation with only weakening of the walls and no hernia sac can be called a "prolapse", the suffix for prolapse (or hernia sometimes) is [-ocele].

There are also "internal' hernias, between bodily compartments. Examples are:

• Esophageal hiatus hernia: Known as a "hiatal hernia", this hernia is a protrusion of a peritoneal sac with abdominal visceral content into the thorax.
• Perineal hernia: The protrusion of abdominopelvic content into the perineal region through a defect in the pelvic diaphragm (levator ani)

A hernia is usually named for the superficial region where it protrudes. An example of this would be a femoral hernia, which starts as an abdominopelvic extrusion, but it ends protruding in the area of the thigh (femoral region). Abdominal or ventral hernias are named according to the abdominal region through which they protrude.

in older times the word "rupture" was used as a synonym for "hernia", as can be seen in a letter written by Dr. Ephraim McDowell in 1829. The image shows an example of an indirect inguinal hernia. 

Original Image courtesy of: nih.gov