A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders and masses of small solids. It can also be used for structural applications; a hollow pipe is far stiffer per unit weight than the solid members.

In common usage the words pipe and tube are usually interchangeable, but in industry and engineering, the terms are uniquely defined. Depending on the applicable standard to which it is manufactured, pipe is generally specified by a nominal diameter with a constant outside diameter (OD) and a schedule that defines the thickness. Tube is most often specified by the OD and wall thickness, but may be specified by any two of OD, inside diameter (ID), and wall thickness. Pipe is generally manufactured to one of several international and national industrial standards. While similar standards exist for specific industry application tubing, tube is often made to custom sizes and a broader range of diameters and tolerances. Many industrial and government standards exist for the production of pipe and tubing. The term "tube" is also commonly applied to non-cylindrical sections, i.e., square or rectangular tubing. In general, "pipe" is the more common term in most of the world, whereas "tube" is more widely used in the United States.

Both "pipe" and "tube" imply a level of rigidity and permanence, whereas a hose (or hosepipe) is usually portable and flexible. Pipe assemblies are almost always constructed with the use of fittings such as elbows, tees, and so on, while tube may be formed or bent into custom configurations. For materials that are inflexible, cannot be formed, or where construction is governed by codes or standards, tube assemblies are also constructed with the use of tube fittings.

Additionally, pipes are used for many purposes that do not involve conveying fluid. Handrails, scaffolding, and support structures are often constructed from structural pipes, especially in an industrial environment.

The first known use of pipes was in Ancient Egypt. The Pyramid of Sahure, completed around the 25th century BC, included a temple with an elaborate drainage system including more than 380 m (1,247 ft) of copper piping.

During the Napoleonic Wars Birmingham gunmakers tried to use rolling mills to make iron musket barrels. One of them, Henry Osborne, developed a relatively effective process in 1817 with which he started to make iron gas tubes ca. 1820, selling some to gas lighting pioneer Samuel Clegg.

When steel pipes were introduced in 19th century, they initially were riveted, and later clamped with H-shaped bars (even though methods for making weldless steel tubes were known already in the 1870s), until by the early 1930s these methods were replaced by welding, which is still widely used today.

There are three processes for metallic pipe manufacture. Centrifugal casting of hot alloyed metal is one of the most prominent processes.^{[citation needed]} Ductile iron pipes are generally manufactured in such a fashion.