The Collaborative International Dictionary
Net \Net\, a. [F. See Neat clean.]
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Without spot; pure; shining. [Obs.]
Her breast all naked as net ivory.
--Spenser. Free from extraneous substances; pure; unadulterated; neat; as, net wine, etc. [R.]
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Not including superfluous, incidental, or foreign matter, as boxes, coverings, wraps, etc.; free from charges, deductions, etc; as, net profit; net income; net weight, etc. [Less properly written nett.]
Net tonnage (Naut.), the tonnage of a vessel after a deduction from the gross tonnage has been made, to allow space for crew, machinery, etc.
Wikipedia
Net tonnage (often abbreviated as NT, N.T. or nt) is a dimensionless index calculated from the total moulded volume of the ship's cargo spaces by using a mathematical formula. Defined in The International Convention on Tonnage Measurement of Ships that was adopted by the International Maritime Organization in 1969, the net tonnage replaced the earlier net register tonnage (NRT) which denoted the volume of the ship's revenue-earning spaces in "register tons", units of volume equal to . Net tonnage is used to calculate the port duties and should not be taken as less than 30 per cent of the ship's gross tonnage.
Net tonnage is not a measure of the weight of the ship or its cargo, and should not be confused with terms such as deadweight tonnage or displacement. Also, unlike the net register tonnage, the net tonnage is unitless and thus can not be defined as "tons" or "net tons".