In medicine, a flexible tube or tubes that are inserted through a narrow opening into a body cavity, particularly the bladder, for removing fluid: One of the main definitions of catheter is that it is a tube which is passed into the body for evacuating fluids or for injecting them into body parts for the administration of diagnostic or therapeutic agents.

Catheters may be made of elastic, elastic web, rubber, glass, metal, or plastic.

A tube inserted into an artery to measure pressure, remove blood, inject medication or a radiographic contrast media, and be used to perform an interventional radiological procedure: Jim's doctor told him that a long slender flexible tube or arterial catheter would be used for inserting into one of his arteries so the abnormal narrowing that he has can be expanded because his condition could reduce blood flow and even cause tissue damage.

A kind of flexible sac with one or more of them being used to apply pressure on an organ or vessel and which functions either for draining fluid from or injecting it into a part of the body for therapeutic purposes: A kind of balloon catheter, with a sausage-shaped balloon, is used to expand narrow arteries which can avoid the need for surgery; although the narrowing of the vessel may occur again.

Another application of the balloon catheter is to treat the bleeding of enlarged veins in the esophagus or stomach, a life-threatening condition of some kinds of liver disease.

A flexible plastic tube that is inserted into the hollow organ located in the lower abdomen which stores urine: Dr. Atherton inserted a bladder catheter into his patient's bladder so continuous urinary drainage could take place.

A long, fine, flexible tube designed for insertion into a peripheral blood vessel through which it can then be manipulated under X-ray control into the heart: The cardiac catheter is used for registration of pressures, sampling of blood, or rapid injection of radiopaque substances which will outline the chambers and blood vessels of the heart.

Blood poison that occurs during an intravenous insertion of a tube to carry fluids into or out of the body: The sterilization unit of the hospital emphasizes procedures to avoid any catheter sepsis which could be caused by their medical instruments.

A long, thin, hollow surgical instrument, that is inserted into a space in the body: A filiform-tipped catheter is one with the leading end that is very slender and flexible in order to facilitate the passage of the catheter through a constricted, angulated, or obstructed tubular structure.

A self-retaining tube that is held in place in the urinary bladder with an inflated balloon: A Foley catheter is constructed so it will stay where it is placed by a surgeon or another medical person in order to diagnose or to treat an ailment.