Dark Force Defense League: An organization of which Gilderoy Lockhart is an honorary member. It may be some sort of vigilante group.
Dark Forces, The: A Guide to Self-Protection: The first-year textbook for Defense Against the Dark Arts.
Davies, Roger (PoA ch. 14): Captain of the Ravenclaw Quidditch team.
Etym: From the Hebrew name David, meaning "beloved". Sarah Emily Davies (1830-1921) was a pioneer in the movement to allow women into colleges, and was the founder of Girton College, Cambridge.
Davy Gudgeon [David]:
Etym: Hebrew, meaning "beloved". St. David is the patron saint of Wales.
Dean Thomas: Etym: From Middle English dene "valley", or deen, a borrowing of a word that ultimately meant "a leader of ten men".
deathday: The date on which a ghost comes into being, or the anniversary of that date. Like a birthday, a cause for celebration.
Death Eaters: Voldemort's inner circle during his reign of terror, now mostly gathered back to the cause. The Death Eaters mentioned so far, and their current statuses, are:
Dead: Bartemius Crouch junior, Evan Rosier, Wilkes
Renounced Voldemort: Igor Karkaroff, Severus Snape
Imprisoned: Antonin Dolohov, the Lestranges, Mulciber, Travers
Alive and free: Avery, Ludo Bagman, Crabbe senior, Goyle senior, Walden Macnair, Lucius Malfoy, Nott senior, Peter Pettigrew
Unknown: Augustus Rookwood
Death Omens: What to Do When You Know the Worst is Coming: A book in Flourish and Blotts.
Decree for the Reasonable Restriction of Underage Wizardry: A law dating from 1875 which disallows student wizards from using magic outside of school.
Dedalus Diggle: Etym: Alternate spelling of Daedalus, of course.
Defense Against the Dark Arts: A required subject for all Hogwarts students, but by far the most hazardous class to teach. Teachers so far:
Harry's first year: Professor Quirrell
Second year: Gilderoy Lockhart
Third year: Remus Lupin (with a bit of substitute teaching from Professor Snape, who is thus the only person we've seen fully survive the job so far)
Fourth year: Bartemius Crouch disguised as Alastor Moody
Deflating Draft: The antidote to a Swelling Solution.
Delacour, Fleur: The Beauxbatons entry in the Triwizard Tournament. Wand: 9.5", inflexible, rosewood and veela hair (one of her grandmother's).
Etym: French for "of the court". A "flower of the court" would be a particularly striking noble lady.
Delacour, Gabrielle: Fleur Delacour's little sister.
Etym: See above.
Delaney-Podmore, Sir Patrick (CoS ch. 8): The head of the Headless Hunt.
Etym: Delaney may be from a French root meaning "wool" or "alder grove", or the Gaelic patronymic Ó Dubhshláine, composed of the elements dubh "black" + slán "challenge, defiance".
Podmore is of uncertain origin, possibly from Middle English pod or pad "frog" + more "fen, marsh".
Deletrius: An incantation to banish a summoned creature.
Etym: An alteration of deleterious, maybe.
dementor: A corpselike, possibly undead creature that feeds on emotions, draining away a person's will to live. Usually swathed in a cloak, their faces are seen only by people about to receive a dementor's kiss. Dementors are the guards of Azkaban.
Etym: Invention from demented, as in cause to be.
dementor's kiss: How a dementor sucks out a person's soul, leaving them in a permanent vegetative state.
de Mimsy-Porpington: see Mimsy-Porpington.
Dennis (PS ch. 3): Part of Dudley Dursley's gang.
Etym: Derived from Dionysius, though the exact etymology is uncertain. St. Denis is a patron saint of France, who according to legend was martyred by decapitation. He is portrayed in art as a headless living figure.
Dennis Creevey: Etym: See above.
Densaugeo: Incantation for a curse that makes the target's teeth grow.
Etym: Latin dens "tooth" + augeo "I augment". To make grammatical sense, ought to be dentemaugeo ("tooth" as direct object) or densauge (imperative: "tooth, grow!").
Department for the Regulation and Control of Magical Creatures: One of the many departments of the Ministry of Magic.
Department of International Magical Cooperation: Another department of the Ministry of Magic, responsible in part for overseeing the Quidditch World Cup and the Triwizard Tournament.
Department of Magical Catastrophes: Another arm of the Ministry of Magic.
Department of Magical Games and Sports: Yet another tentacle of the Ministry of Magic.
Department of Magical Transportation: A section of the Ministry of Magic which, among other things, regulates Apparation.
Department of Mysteries: Another part of the Ministry of Magic. Appropriately, we as yet have no idea what it does.
Derek (PoA ch. 11): A Hogwarts student, two years behind Harry, house unknown.
Derrick (PoA ch. 15): A Beater on the Slytherin Quidditch team.
Etym: Related to Theoderic, "people-rule". As a common noun, from the surname of a noted hangman at Tyburn, circa 1600.
Dervish and Banges: A toy shop in Hogsmeade.
Etym: Dervish is from Arabic darwiish, meaning a member of a Sufi fraternity, and is also used in fantasy to mean a whirling supernatural being. Banges would be an alteration of bangs.
Devil's Snare: A magical plant which grabs hold of anyone within range. (What it plans to do with them next has not been recorded.) It recoils from bright light.
Devon: A region of the UK which is home to the Flamels.
Diagon Alley: A wizard shopping district somewhere in London, accessible from The Leaky Cauldron and the Floo Network. Establishments therein include Eeylops Owl Emporium, Flourish and Blotts, Gringotts, Madam Malkin's Robes for All Occasions, Gambol and Japes, Florean Fortescue's ice cream parlor, Magical Menagerie, Quality Quidditch Supplies,and Ollivander's.
Diffindo: The incantation for the Severing Charm.
Etym: Possibly Latin, "I split, cleave". Imperative: diffindere.
Diggle, Dedalus: A wizard who, according to Professor McGonagall, "never had much sense". Harry met him at The Leaky Cauldron.
Etym: Probably from the word dighel, meaning secret or obscure, though the proper etymology of the surname is different.
Diggory, Amos (GoF ch. 6): Cedric Diggory's father. He works for the Department for the Regulation and Control of Magical Creatures.
Diggory, Cedric: A Hufflepuff, two years ahead of Harry, the captain and Seeker of the Hufflepuff Quidditch team and one of the Hogwarts champions in the Triwizard Tournament. Also a co-winner of the tournament, which inevitably leads to him being killed seconds later. Now exists as a shade. Wand: 12.25", springy, ash and unicorn hair.
Etym: Goes back to the medieval romance of Sir Degaré, and is probably from French égaré "strayed, lost".
Dimitrov (GoF ch. 8): A Chaser on the Bulgarian national Quidditch team.
Etym: From Greek Deemeetrios "(follower) of Demeter".
Dippet, Armando (CoS ch. 13): The headmaster of Hogwarts in Tom Riddle's time, probably Dumbledore's immediate predecessor.
Etym: Variation of Theobald, from the Germanic personal name *þeudo "people, race" + bald "bold, brave".
Disapparation: Apparation as seen from the point of departure.
Disarming Charm: A spell that knocks an opponent's wand out of their grip. Incantation: Expelliarmus.
Dissendium: The password to open the secret passage from Hogwarts to Honeydukes.
Etym: Latin, "dissension, discord".
dittany: OED lists a number of possible plants but, for our purposes, probably Origanum dictamnus aka Dictamnus creticus, once alleged to have medicinal virtues.
Divination: An elective subject at Hogwarts, taught, in a loose sense of the word, by Professor Trelawney.
Dobbs, Emma (GoF ch. 12): A Hogwarts student three years behind Harry, house unknown.
Etym: From a variation of Robert, which comes from Old English/German hrothi "fame" + berhta "bright".
Dobby: A house-elf formerly in the employ of Lucius Malfoy, who went to Hogwarts to break new ground with the unthinkable practice of being paid to work.
Etym: Word for a household sprite or apparition, particularly a brownie. See house-elf for more.
Dolohov, Antonin (GoF ch. 30): A Death Eater, imprisoned in Azkaban.
Etym: No info on the name and the nearest Russian words don't look too likely at the moment.
Doris Crockford: Etym: The name of a sea nymph in Greek myth, origin unknown. Noted as a given name only as far back as 1819.
Dot [Dorothy] (GoF ch. 1): The cook at The Hanged Man.
Etym: The name is believed to have developed from an arbitrary inversion of Theodora, Greek for "God's gift".
Draco Malfoy: Etym: Latin for "dragon", though the direct source for Mr. Malfoy's name is probably the Athenian ruler who gave us the word draconian.
dragons: Many dragon species are still found in the wild throughout Europe. All appear to be are variations on the typical European dragon, with wings and fiery breath. Specific varieties mentioned so far are the Chinese Fireball, Common Welsh Green, Hebridean Black, Hungarian Horntail, Swedish Short-Snout, and Norwegian Ridgeback.
Although captive dragon breeding is now outlawed, dragons are still useful to wizards in many ways. Their heartstrings are used in wands, their dung is used as compost for magical plants, and Albus Dumbledore alone has invented twelve uses for their blood.
When facing a dragon, it is helpful to remember that its eyes are its weakest spot.
Dragon Breeding for Pleasure and Profit: One of Hagrid's collection of dragon books.
Dragon Keeper's Guide, A: Another of Hagrid's collection of dragon books.
Dragon Species of Great Britain and Ireland: Yet another of Hagrid's collection of dragon books.
Draught of Living Death: An extremely powerful sleeping potion.
Dreadful Denizens of the Deep: A book in the Hogwarts library.
Dr. Filibuster's Fabulous Wet-Start, No-Heat Fireworks: Magical fireworks available at Gambol and Japes.
Drooble's Best Blowing Gum: A wizard candy. Probably best to steer clear of it, because...
Etym: One of a number of variations on the French word for "trouble".
Drought Charm: A spell to lower or dry up small bodies of water.
Dudley Dursley: Etym: From an Old English placename, composed of the name Dudda + leeah "wood, clearing", and maybe a pun on him being a bit of a dud as a human being.
Dumbledore, Aberforth: Albus Dumbledore's brother, banned from using magic after being caught "practicing inappropriate charms on a goat".
Etym: See below.
Dumbledore, Albus: A bowling and chamber music aficionado, a gourmet of candies, and the headmaster of Hogwarts. Has been known to dabble in projects useful to wizard society, such as discovering the 12 current uses for dragon's blood, and defeating the dark wizard Grindelwald. Said to be an ex-Gryffindor.
Etym: Old word for a bumblebee, from dumble "stupid, dull, slow" + dor "insect that makes a loud humming noise".
Dundee: A city in northern Scotland.
Durmstrang Institute: Another school of magic, thought to be somewhere in northeastern Europe. Its curriculum is rumored to include dark magic.
Etym: Almost certainly invented from the German phrase sturm and drang.
Dursley, Dudley: Harry's cousin, a fine young lad who knows how to get the best out of life, at least from people who are swayed by screaming tantrums.
Etym: Stated by Rowling to be from the place name, which for the record is from the Old English given name Deeorsige + leeah "wood, clearing".
Dursley, Marge: Harry's aunt-in-law, Vernon Dursley's sister, dog fancier, occasional babysitter, and dispenser of time-honored wisdom on the subject of breeding.
Etym: See above.
Dursley, Petunia: Harry's aunt, Lily Potter's sister, who selflessly gave up several cubic feet of storage space to house her weird nephew.
Etym: See above.
Dursley, Vernon: A respectable businessman, the director of Grunnings, a doting father, and really not at all to blame for his nephew being a freak.
Etym: See above.