NAME ORIGINS

Alastor - Greek God who sought revenge on people.
Albus - In Latin means white (maybe for white beard). Wisdom. Or.. Governor of Britain at the death of the emperor Pertinax, Decimus Clodius Albinus (Albinus=Albus?) attempted to seize the throne but ended up as Caesar in alliance with another imperial contender, Septimius Severus. After Severus defeated two other rivals (Voldemort and... maybe Slytherin?), the now expendable Albinus was forced into another attempt at usurpation, an attempt that came to an end at the bloody battle of Lyon.
Aragog - "Arachnid" means spider.
Argus - In Greek mythology, Argus was a monster that had a hundred eyes and was ever-so-watchful.

Arthur - Could represent King Arthur. The legend presents Arthur as a leader in ancient times who defeats the Saxons and other enemies. He thereby unites the people of Britain in peace and harmony.
Bagman - A person who collects money, as for racketeers.
Beauxbatons - French for "beautiful wands".
Bellatrix - A pale yellow star indicating the left shoulder of the constellation Orion, the Great Hunter. Bellatrix is known as the "Female Warrior."
Bode - To be an omen. Also, a stop or delay.
Boggart - "Baggart" (pronounced the same), an Irish word, means "a threat".
Charlie - A diminutive of Charles, which means manly and strong.
Cho - Japanese for "butterfly"
Colin - Means youth, child, or victor.
Cornelius - See Lucius.
Dolores - lady of sorrows
Draco - Draco is a constellation that looks like a dragon but is a snake. In Latin, Draco means "dragon."
Dumbledore - Means "Bumblebee" in Old English.
Durmstrang - "Sturm und drang" is German phrase meaning storm and stress.
Dursley - A town near J.K. Rowling's birthplace.
Figg, Mrs. Arabella - "Fig" means "not literal" and a fig leaf is something that conceals or camouflages. Hmm...
Filch - To "steal."
Firenze - Italian name for the city of Florence.
Flitwick - A town in England.
Fleur Delacour - Means "Flower of the Court" in French.
Fluffy - "Cerberus" the three-headed dog was the guardian of the underworld in Greek mythology.
Fudge - "Fudge," besides being a delicious chocolate confection, can mean nonsense. As a verb, it means to evade or to falsify. In technological jargon, it means "to perform in an incomplete but marginally acceptable way."
Gilderoy - A highwayman known for being handsome. May also come from the word "gilded," which is defined as having a pleasing, showy appearance, which covers something of little worth. The name "Roy" is old Old French for "regal one" or king.
Hedwig - A saint that lived in Germany in the 13th and 14th centuries Means "refuge in battle."

Hermes - The Greek Messenger.
Hermione - Means well born or stone. Feminine version of Hermes. In Greek Mythology, she was the daughter of Helen of Troy and King Menelaus of Sparta. Hermione is also a character in Shakespeare's A Winter's Tale.
Lily - Pure.
Lockhart - Town in Australia near Wagga Wagga ("Compose a poem about my defeat of the Wagga Wagga Werewolf"?).
Lucius - Similar to Lucifer (the devil).
A Roman General named Lucius Cornelius Sulla was usurped by the people of Rome, but defeated them and seized control as a dictator. After doing so, he removed most of the popular say in the government and returned it to the Senate of Rome, which controlled the people, and founded a firm Republic. (Thanks to Rosemary for this great theory!)
Luna - The Roman goddess of the moon. Also, it is a term for silver in alchemy.
Lupin - Lunar means moon. Lupin means wolf-like. Canis Lupus is the scientific name for wolf.
Malfoy - "Mal foi" means "bad faith" in French. Malfoy has "mal" in it, meaning evil.
Marietta - "Little Bitter."
Minerva - In Roman Mythology was the Goddess of Wisdom. In Greek Mythology she was the Goddess of War. She gave strict punishments.
Mirror of Erised - Erised backwards is desire (as in "you'll see what you desire"). The inscription around the top of the Mirror of Erised, if shown backwards with the spaces rearranged, says: I show not your face but your hearts desire.
Moody - Not in a good mood.
Mundungus - A stinking tobacco.
Nagini - "Naga" is snake in Sanskrit.
Neville - Old French for "from the new farmland."
Nicholas Flamel - Was a real alchemist, and supposedly DID create the Philosopher's Stone (not "Sorcerer’s"). The tale was that he had spent decades of his life trying to create the Philosopher's Stone, which could turn any metal in to gold and unlock the secrets to immortality, but could not figure it out.
Nimbus - "Nimbus" means "cloud." Nimbus was also a god in Greek Mythology.
Norris, Mrs. - A character in one of J.K. Rowling's favorite author's (Jane Austen) books.
Parvati - Parvati is a Hindu Goddess married to Hindu God Siva the Destroyer. She gave birth to a baby boy named Ganesh, who Siva beheaded, but replaced the old head with an elephant head after Parvati reamed him out. Sister of the Goddess of the Ganges, Padma. "Patel" is a common Indian last-name.
Peeves - "Peeve" means "little devil" or something that gets on your nerves (like a pet peeve).
Percival - One of the legendary Knights of the Round Table. The name itself means "pierces the valley" or "destroyer."
Quibbler - To quibble means to evade the truth or importance of an issue by raising trivial distinctions and objections. A quibble is an archaic term for a pun.
Remus - Brother of Romulus (founder of Rome). They were raised by a female wolf. He was killed by Romulus.
Ron - Interesting when taken in conjunction with Arthur: advisor to the king.
Ronan - An Irish saint.
Severus - Sever means "to cut off". Severe also means cruel, strict. In ancient history, Lucius Septimius Severus restored stability to the Roman empire after the tumultuous reign of the emperor Commodus (See Albus) and the civil wars that erupted in the wake of Commodus' murder (if you watched "Gladiator" then you should know this).
Sibyll - Sibyll comes from the Sibyls, who were famous prophets in ancient mythology.
Sirius - Named after the star Sirius, also known as the Dog Star. Seen during summer "dog" days.
Skeeter, Rita - "Skeeter" is short for "mosquito." As most people can attest, mosquitoes are among the most annoying life-forms on this planet.
Snape - A town in England, also based after a person JKR knew.
Tom Marvolo Riddle - If you rearrange the letters it spells: "I am Lord Voldemort."
Umbridge - Sounds like "umbrage," which is a feeling of anger caused by an offence. While it can also mean "shadow" or "offense," it can also mean "jealous of another, as standing in one's light or way."
Voldemort, Lord - There was a dark wizard in medieval times named Voldermortist. In another language, Voldermortist means "Lord of Evil" or "Dark Lord". Legend has it that Voldermortist once tried to destroy Merlin before the time of King Arthur (Mr. Weasley?), by bewitching good people, and simply bribing those who already were evil. Legend has it that Merlin destroyed Voldermortist by using a simple paralyzing charm (full body bind?), fed him to the many-headed-beast (Fluffy?) of the lake, the Lady of the Lake's pet (Giant Squid?), freed the bewitched people, and destroyed the evil men. That was maybe twelve, thirteen years before Arthur (how long it was from Voldemort's destruction until Harry started Hogwarts). In French means "thief of death" (meaning escaping death).

Spells

Accio (summoning charm) - Latin for "to summon."
Avada Kedavra (killing curse) - Aramaic phrase that means "may it be destroyed."
Avis (spell that Ollivander used to make birds fly out of Krum's wand) - Latin for "bird."
Cunjunctivitus Curse (spell that Krum used to "do something" to the eyes of the Chinese Fireball during the first task of the TWT) - Conjunctivitis is the scientific name for pink-eye -- the illness that children often get that makes eyelids crust together.
Cruciatus Curse - "Crucio" is Latin for "to torture."
Deletrius (banishing spell) - Latin for "to erase."
Densuageo (spell that Malfoy used to make Hermione's front teeth grow huge) - "Dens" is Latin for "teeth." "Augeo" is Latin for "to enlarge."
Diffindo (spell Harry used to cause Cedric's bag to split apart) - In Latin, "Diffundo" means "scatter."
Ennervate (used to revive someone who has been stunned) - When spelled with two n's, as ennervate, it actually means to add nerve (daring or strength). With one n, as enervate, it means to weaken. The spell in the books is "ennervate," so it makes perfect sense.
Expelliarmus (disarming spell) - Latin combination "to expel" and "arma" (weapon).
Impedimenta (used to slow down attackers) - "Impedio" is Latin for "to hinder."
Imperius Curse - "Impero" is Latin for "to control" and "imperium" is Latin for "absolute control."
Lumos (used to make a wand emit light) - "Lumen" is Latin for "light" and "luminous" means "emitting light" in English.
Morsmordre (makes the Dark Mark appear in the sky) - Combination between "mors" (Latin for "death") and "modere" (Latin for "to bite"). Death bite? No -- Death Eater.
Nox (cancels out Lumos) - "Nox" is Latin for "darkness."
Orchideous (makes flowers spring out of the tip of a wand) - Orchid is a type of flower.
Patronus (dementor defense spell) - "Patronus" is Latin for "protector."
Quietus (cancels out "Sonorus") - Obviously "quiet" means to be silent, and "quietus" in Latin means to be quiet.
Reducio (shrinking charm) - "Reduce" means "to make smaller" in English.
Riddikulus (spell used to transform a scary Boggart into a humorous shape) - "Ridiculous" means "absurd."
Sonorous (spell used to magnify a voice like a loudspeaker) - "Sonorus" is Latin for "loud."
Stupefy (spell used to make someone unconscious) - "Stupefy" in English means "dull the senses of; daze."
Veritaserum (truth potion) - "Veritas" is Latin for "truth" and a serum is a potion.

THANK YOU TO SUPERHPFAN FOR THESE ORIGINS!