National Pipe Thread [1]

A United States standard for screw threads cut in a piping and fittings for general use. The National pipe thread (NPT) standard calls for the diameter of the threads to taper, so that the piping is screwed into a mating fitting, the fit of the mal and female threads becoming increasingly tight.

A sealant must be used with NPT joints to make them liquid or air tight.

See also PEI/RP300 [2].