Failure or insufficiency in the peristalsis movement of the gastrointestinal tract which normally moves GI contents from mouth to anus.
Thought to be caused by a disruption in either the neural and/or neuro-hormonal reflexes that normally regulate bowel motility.
Multiple causes possible - likely an inflammatory condition
Opioid-induced
Post-operative (irritation +/- opioids)
Electrolyte imbalance (hypokalemia, hypomagnesemia)
Colonic pseudo-obstruction (aka Ogilvie's syndrome)
Gallstone<unk> , pancreatitis
Peritonitis (e.g. from infection, malignancy, trauma)
Clostridium difficile colitis
Burns, acute radiation syndrome
Spastic<unk>  (rare: porphyria or lead poisoning)
Opioid-induced
Post-operative (irritation +/- opioids)
Electrolyte imbalance (hypokalemia, hypomagnesemia)
Colonic pseudo-obstruction (aka Ogilvie's syndrome)
Gallstone<unk> , pancreatitis
Burns, acute radiation syndrome
Spastic<unk>  (rare: porphyria or lead poisoning)
Ileus
Small bowel obstruction
Acute pancreatitis
See constipation, abdominal pain
Short bowel syndrome
Acute gastroenteritis 
Acute gastroenteritis
Chronic pancreatitis