A pipe is round tubular section or hollow cylinder used mainly to convey media. It can also be used for structural applications. In layman's terms the appellations pipe and tube are almost interchangeable, but in industry and engineering discipline the terms are uniquely defined. Depending on the applicable standard to which it is manufactured, pipe is specified by the internal diameter (ID) and a wall thickness, a nominal diameter and a wall thickness, or an outside diameter (OD) and a wall thickness. Tube is most often defined by the outside diameter (OD) and a wall thickness but may be specified by any combination of dimensions (OD, ID, wall thickness). Pipe is generally manufactured to several long-standing and broadly applicable industrial standards (such as ASME/ANSI B36.10/B36.19). 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, the term "tube" is more widely used in the United States, whereas "pipe" is more common elsewhere in the world.
Both "pipe" and "tube" imply a level of rigidity and permanence, whereas a hose is usually portable and flexible. Pipe assemblies are almost always constructed with the use of fittings such as elbows, tees, etc., 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