tube

A tube is a hollow cylindrical object used to convey or contain fluids, gases, or powders, or to serve as a structural member. Tubes are manufactured from metal, plastic, glass, or composites and come in a range of outside diameters and wall thicknesses. In industry, tube sizes are often specified by outside diameter and wall thickness; pipes are typically described by inside diameter or nominal bore. The term also denotes the geometric form of a hollow cylinder.

Tubes occur in many forms: rigid tubes used in plumbing, HVAC, and automotive systems; flexible tubing for hoses and medical devices; and glass tubes such as test tubes used in laboratories. In consumer packaging, toothpaste and cosmetics can be dispensed from squeeze tubes. Tubes can be seamless or welded, and may be lined or coated to resist corrosion or wear.

In biology and medicine, many tubular structures are described as tubes, such as the esophagus, trachea, bile ducts, and intestines. Tympanic tubes (grommets) are small tubes inserted into the eardrum to ventilate the middle ear. Medical tubes include catheters, drainage tubes, and endotracheal tubes.

In technology, vacuum tubes were used for amplification and switching in early electronics, including radios, audio amplifiers, and computers. Cathode-ray tubes powered mid-20th-century displays before being largely replaced by solid-state devices and flat-panel screens. The word tube remains in historical and specialized contexts, and in some product names and brand usage.