A pipe is a tubular section or hollow cylinder, usually but not necessarily circular cross-section, used mainly to convey substances which can flow liquids and gases (fluids), slurries, powders and masses of small solids. Can also be used for structural applications; hollow pipe is far stiffer per unit weight than 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 generally specified by a nominal diameter with a constant outside diam (OD) and a schedule that defines the thickness.

Pipes vary in materials from which they are made

Proper selection of pipe largely affect the reliability of the pipeline operation and significantly increases its service life. Pipes must be durable, resistant to soil, groundwater and ground currents, withstand the internal pressure of the water, have a smooth inner surface and waterproof walls, as well as, importantly, affordable price. Produced modern pipe industry to a greater or lesser extent correspond to requirements imposed on them.