Etym: Naga is the name of a Hindu deity represented as a snake, and various other mythological snake-creatures; this name, I think, signifies someone who has transformed (or been transformed) into a snake.
Narcissa Black Malfoy: Etym: The feminine form of Narcissus, the figure from Greek legend who gave his name to a flower after pining away from falling in love with his reflection. The flower signifies egotism or self-esteem.
nargles (OotP ch. 21): A creature which Luna Lovegood believes can infest mistletoe.
Etym: No idea. Narghile is another word for a hookah, but I don't think that's it.
Nastily Exhausting Wizarding Tests: The highest-level graduation exams that Hogwarts offers; equivalent to the "A-level" (advanced level) exams in real British schools. Possible passing grades are O(utstanding), E(xceeds Expectations), and A(cceptable); failing, P(oor), D(readful), and T(roll).
Natalie McDonald: Etym: From Latin natale (domini), i.e. Christmas Day.
Nature's Nobility: A Wizarding Genealogy (OotP ch. 6): A book in the Black house.
Nearly Headless Nick: The students' nickname for Sir Nicholas de Mimsy-Porpington.
Nettles, Madam Z., of Topsham: A witch quoted by the Kwikspell ad copy.
nettle wine: Believe it or not, somebody out there actually makes this...
Neville Longbottom: Etym: From a two French towns of the same name, or an Anglicized form of Ó Niadh, from a name meaning "warrior". The name of various earls; also the first Lord Latimer of the village of Snape. The Battle of Neville's Cross, October 17, 1346, was a notable English victory over the Scots, who were allied with France against the English.
N.E.W.T.s: Nastily Exhausting Wizarding Tests.
Etym: From Old English neowe "new" + tun "enclosure, settlement".
newts, double-ended: Creatures available at the Magical Menagerie. The reader is left to speculate on which end they have two of.
New Theory of Numerology (OotP ch. 23): A book Harry gave Hermione for Christmas.
Nicholas Flamel: Etym: See below.
Etym: St. Nicholas, Bishop of Myra circa 300, is the patron saint of sailors, pawnbrokers, wolves, and scholars, especially schoolboys.
niffler: A small, furry black creature native to Britain, which has an ability to sniff out treasure. It can be trained to bring anything it finds back to its handler.
Nigellus, Phineas (OotP ch. 6): Sirius Black's great-great-grandfather, a Slytherin alleged to be the least popular headmaster Hogwarts ever had.
Etym: Latin, diminutive of "black".
Nimbus Two Thousand: The (once) most advanced broom yet created by modern magical technology.
Nimbus Two Thousand and One: The (previously) new most advanced broom yet created by modern magical technology.
nogtail (HBP ch. 7): A beast found in Norfolk and hunted by wizards.
Etym: Another version of the noggle, nuggle, or nygel, mythical shapeshifters of evil intent found in various folklore throughout Britain.
Norbert: A Norwegian Ridgeback hatched by Hagrid, raised in secret to the age of a month or two, then spirited away to Romania to finish growing up in the wild and probably enter a lifetime of psychotherapy.
Etym: St. Norbert of Xanten (1080-1134), was the founder of the Premonstratensians (or Norbertines, or White Canons), a monk-like order.
Norberta: Norbert's new identity. Clearly the adjustment issues were worse than we first thought.
Norfolk: A rather flat, boring sector of England, I am told. I can't even find a decent Web site on it...
Norris, Mrs.: Filch's cat and assistant in spotting trespassers.
Etym: Named for a sneaky, spiteful character in the Jane Austen novel Mansfield Park.
Norwegian Ridgeback: A species of dragon, sporting mildly poisonous fangs in addition to the usual features. Its eggs are large and black.
Nose-Biting Teacup: A gag item from Zonko's Joke Shop.
Nosebleed Nougat (OotP ch. 6): An ingredient in the Skiving Snackboxes.
Notable Magical Names of Our Time: A book in the Hogwarts libary.
Nott, Theodore: A Slytherin in the same year as Harry.
Nott senior (GoF ch. 33): A Death Eater, still at large.
Etym: See above.
Nox: An incantation that cancels out Lumos.
Etym: Latin, "night".
Numerology and Gramatica: A textbook for one of Hermione's classes in her third year.
Nurmengard (DH ch. 18): Grindelwald's current residence, originally built for the benefit of his enemies.
Etym: Well, first consider that -gard is roughly equivalent to -berg, shuffle a few consonants, and voilà!
Nymphadora Tonks Lupin: Etym: -dora as a name element usually means "gift"; this could be "nymph's gift" or "girl who is a gift". The mythical Andromeda had six sons and a daughter; if that's the case with Mother Tonks, the latter meaning could be it.