In planetary classification a class N planet is one type of world listed in the scale.
In particular, a class N(2) planet is noted as being the place to use a specialized non-oxidizing explosive called tri-nickolas powder. (TNG: "Night Terrors")
Background
Information taken primarily from Star Trek: Star Charts indicates this is a "reducing" planet having an age that ranges from 3 to 10 billion years, and a diameter between 10,000 and 15,000 km. Star Charts mentions reducing planets are located within the "ecosphere" of a star system, with class N planets catagorized by a high surface temperature due to greenhouse effect (which causes all water on the planet to exist only as vapor) and an extremely dense atmosphere, comprised of carbon dioxide and sulfides. If life can exist on a class N planet, it is unknown in what form(s) it may appear, but an example given of a Class N planet is Venus.