National Bank Note Company - a U.S. printer authorized to print securities and holder of the contract to print U.S. postage stamps from 1861 to 1873. Like the Continental Bank Note Company stamps the Nationals were printed on a hard white paper, helping to distinguish them from the American Bank Notes that followed. Many of the National Bank Notes had "grills". Grills are found on the 1861 Issue (1867-1869), the 1869 Pictorials, and the early 1870 Bank Notes. 

National Defense Issue - (Scott’s 899-901) a set of stamps promoting national defense printed and perforated using the electric eye method in 1940.

National Parks Issue - (Scott’s 899-901) a 1934 set of commemorative postage stamps honoring various National Parks. The National Parks stamps were reissued in imperforate and ungummed sheets in 1935 in an attempt to mollify collectors (see Farley’s Follies).
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National Postal Museum - a division of the Smithsonian Institution, houses the National Philatelic Collection. The National Postal Museum has a great web presence. 

Never hinged (NH) - an unused stamp with full, undisturbed gum and never attached to a stamp hinge. Often used interchangeably, and incorrectly, with mint never hinged. "Mint" carries a further connotation of freshness.

Newspaper and Periodical Stamps - (Scott’s "PR") stamps issued for prepayment of postage on newspapers and periodicals mailed in bulk. First issued in 1865, the last Newspaper and Periodical stamps were sold in 1899.

NG - (No gum) The term "NG" is used with stamps totally lacking in gum and with no apparent cancellation.

NGC - (Natural Gum Crease) an abbreviation used by auction houses to indicate a natural crease, usually caused by gum or paper shrinkage

NGAI - (No Gum As Issued) Certain stamps were issued without gum, particularly some souvenir sheets, reprints and reproductions. 

NGS - (Natural Gum Skip) an abbreviation used by auction houses to indicate a natural gum skip. A natural gum skip is not considered nearly as serious as a gum skip caused by mishandling of the stamp.

Nibbled Perf - a perforation that is shorter than its neighbors, usually the result of careless handling. Similar to a torn perf, but less drastic. If the perf is entirely missing it is a torn perf.

NYFM Cancels (New York Foreign Mail) - fancy cancels used in New York for foreign mail only