@wikipedia


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).


Usually built of metallic, ceramic or plastic material.

Normal pipe has a constant cross-section area , meaning that it is not varying along the pipe length :

A(l) = A = {\rm const} \Leftrightarrow \frac{\partial A}{\partial l} = 0


See Also


Physics / Fluid Dynamics / Pipe Flow Dynamics / Pipeline Engineering