Bagshot, Bathilda (PS ch. 5): Author of A History of Magic.
Etym: A type of ammunition. Looking at this whole name, it appears the history of magic must have been pretty, um, exciting...
Bandon Banshee: Allegedly banished by Gilderoy Lockhart. A banshee is a wailing or singing demonic spirit; Bandon is a town in County Cork, Ireland.
banshee: see Bandon Banshee.
Bane [Banes or Bain]:
Etym: Bunch of choices here: from a nickname meaning "bones"; a Welsh patronymic meaning "anvil"; Gaelic for "white, fair"; Middle English meaning "welcoming, friendly"; or Middle English/Old French meaning "bath". Or perhaps the character name is just from the modern English word.
Baruffio (PS ch. 10): A wizard infamous for misspeaking a charm and conjuring up a buffalo.
Etym: No etymology.
basilisk: A mythical creature supposed to spring from an egg laid by a rooster and incubated by a serpent or toad. Basilisks are usually depicted as snakelike, sometimes with a crown (its name derives from the Greek for "king") and are capable of turning any living thing to stone by looking at them.
The one which had been lurking in the Chamber of Secrets at Hogwarts is additionally in danger from the crowing of roosters, and generates an aura that sends spiders fleeing.
Bathilda Bagshot: Etym: OED has an entry for batilde, an obsolete form of a word meaning "embattled".
Bayliss, Hetty (CoS ch. 5): A Muggle who spotted the flying Ford Anglia.
Etym: From Late Latin baiulus "carrier, porter", cognate to bailiff.
Beater: A member of a Quidditch team who deflects Bludgers away from their teammates (and, ideally, toward the opponents).
beechwood: Various trees of the genus Fagus, all tall, wide-spreading, and preferring temperate climes. The wood is durable under water, and used for cabinetry, tool handles, and shipping containers. Also cultivated as a shade tree.
Beginner's Guide to Transformation, A: The first-year textbook for Transfiguration.
Bell, Katie (PS ch. 12): A Chaser on the Gryffindor Quidditch team.
Etym: Does in fact come from Middle English for "bell".
Bertie Bott's Every Flavor Beans: Jellybean-like magical candies of any flavor imaginable... including the ones you don't want to imagine.
bezoar: This term has been applied to various substances believed to be universal antidotes, but most commonly to a concretion formed around foreign matter in the gut of certain ruminants. Originally these were taken from the wild goat and antelope of Persia, which are also known as the bezoar goat and bezoar antelope.
bicorn: A creature from early French and English literature, it subsisted by devouring virtuous husbands. Powdered bicorn horn is one of the ingredients of the Polyjuice Potion.
Etym: From Old German vilja "will" + helma "helmet".
Binns, (first name unknown): The teacher of the History of Magic classes at Hogwarts, and unusual among history teachers for being as dead as his lectures.
Etym: From the Old English name Binna, of uncertain origin, or a word for an open manger, stall, or hollow place.
Black, Sirius: The person who lent Hagrid his flying motorcycle to take the infant Harry to the Dursleys'.
Etym: Just means black, probably... as a side note, though, some instances of this surname come from Old English blaac, the equivalent of French blanc... meaning "white"!
Black Malfoy, Narcissa: Etym: See below.
Blaise Zabini: Etym: Full etymology unknown but cognate to Blasius, the name of the patron saint of wool-workers and sufferers of throat diseases. Once very popular in England, a major wool producer. ECN: "The only relic of the trade in Romsey, Hants (once a wool staple), is an inn called the Bishop Blaise."
Bletchley (PS ch. 11): The Keeper for the Slytherin Quidditch team in Harry's first year.
Blood-Sucking Bugbear: What Hagrid thought might be killing his roosters. A bugbear in popular legend is a monster in the shape of bear said to devour naughty children.
Bloody Baron: The house ghost of Slytherin.
Bludger: A small, hard ball used in Quidditch. Bludgers fly around the field of play attempting to knock players off their brooms.
Etym: Undoubtedly from bludgeon. OED has an entry for bludger, actually, but in the interest of preserving our G rating we must move on.
bluebell fire: A magically conjured blue fire that can be carried around in a jar.
Body-Bind: A curse that paralyzes the target completely. Incantation: Petrificatus Totalus.
Bones, (first names unknown) (PS ch. 4): The last name of a wizard couple killed by Voldemort.
Etym: Derived via Yiddish from Italian bona "good".
Bones, Susan (PS ch. 7): A Hufflepuff, in the same year as Harry.
Etym: See above.
Bonfire Night: November 5th, in England the commemoration of the Gunpowder Plot. Typically involves lots of fireworks.
Boot, Terry (PS ch. 8): A Ravenclaw, the same year as Harry, and a member of Dumbledore's Army.
Etym: From Middle English for "boot" or Dutch for "boat".
Borgin and Burkes: A shop in Knockturn Alley specializing in the ingredients of dark magic; Evil backwards-R Us.
Etym: No info on Borgin, but perhaps it's supposed to sound like Borgia. The name Burke comes from Old High German burg "fortification". A likelier source for the store's name, however, is the 19th-century murderer and grave-robber William Burke.
Borgin, Mr. (CoS ch. 4): One of the founders of Borgin and Burkes, still working there, or else a descendant.
Etym: See above.
Break with a Banshee: One of Gilderoy Lockhart's books.
Brockelhurst, Mandy (PS ch. 8): A Ravenclaw, the same year as Harry.
Etym: Placename, after a wooded hill that was home to badgers. Brock on its own has been a word for various small animals, including badgers.
brooms: The flying broom is an ancient and venerable mode of wizard transportation, and an indispensable part of Quidditch. Types of brooms include the Cleansweep Five, Comet Two Sixty, Nimbus Two Thousand, Nimbus Two Thousand and One, and Shooting Star.
Brown, Lavender (PS ch. 8): A Gryffindor, the same year as Harry.
Etym: Really means "brown".
budgie: A small yellow bird, very popular as a pet in Britain at one time, less common these days.
bugbear: see Blood-Sucking Bugbear.
Bulstrode, Millicent (PS ch. 8): A Slytherin, the same year as Harry.
Etym: A place name, from Old English burh "fortress, town" or bula "bull" + strood "brushwood".
Burning Day: The day on which a phoenix renews itself.
Burrow, The: The house of the Weasley family.