a pipe is a tube or hollow cylinder for the transport of fluid. the terms 'pipe' and 'tube' can be used for the same objects. 'pipe' is generally specified by the internal diameter (id) whereas 'tube' is usually defined by the outside diameter (od) but may be specified by any combination of dimensions (od, id, wall thickness). 'tube' is often made to custom sizes and may often have more specific sizes and tolerances than pipe. the terms 'tube' and 'tubing' are more widely used in the usa and 'pipe' elsewhere in the world.