Etym: A type of ammunition. Looking at this whole name, it appears the history of magic must have been pretty, um, exciting...
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.
Bathilda Bagshot: Etym: OED has an entry for batilde, an obsolete form of a word meaning "embattled".
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.
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.
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".
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 Comet Two Sixty and the Nimbus Two Thousand.
Brown, Lavender (PS ch. 8): A Gryffindor, the same year as Harry.
Etym: Really means "brown".
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".