So, what exactly is a "hallow?"
I went on a bit of a Harry Potter binge this weekend that included watching all four movies, rereading the first 500 pages of book six, and wondering how in the world I'll control myself until the release of book seven. My HP overdose led me on a search to figure out exactly what the title of book seven, Harry Potter and the Deathly Hallows, means. This is what I came up with.
1) www.dictionary.com provides several definitions for “hallow:”
a. Verb: to make holy; sanctify, consecrate; to honor as holy, consider as sacred, venerate. This is certainly the context in which I have heard the word “hallow” before, or, more accurately, hallowed. For example, in shows like Buffy you always hear about “hallowed ground” meaning a sacred or holy place. So perhaps Harry will be visiting places that are considered sacred or holy in the wizarding world. On the other hand, the word “hallow” in the title is certainly a noun seeing as it is modified by the adjective “deathly.”
b. Interjection: used to call or answer someone, or to incite dogs in hunting. Perhaps Harry is being chased by hunting dogs? Is that why the “hallows” are “deathly?”
c. Noun: the cry “hallo!,” a shout of exultation. I sincerely doubt anyone is excited about the murder and mayhem sure to ensue in book seven.
2) From the online etymology dictionary: from the old English “halgian”—“to make holy, to honor as holy,” related to “halig” meaning “holy” from P.Gmc I don’t know what P. Gmc means, and I took a class on the history of the English language that, among other things, involved extensive readings of etymology in the Oxford English Dictionary. Regardless, this is not helpful as it merely regurgitates the only definition I already knew.
3) Having exhausted my word resources, a Google search then directed me to Wikipedia articles on both Halloween and All Saints’ Day. Wikipedia states, “The festival of All Saints, also sometimes known as “All Hallows” or Hallowmas (“hallows” meaning “saints” and “mas” meaning “mass”), is a feast celebrated in the honor of all the saints, known and unknown.” Well, at least now I can see it used as a noun. Apparently “hallow” is a synonym for “saint.” However, as far as I know, the wizarding community doesn’t revere Roman Catholic saints, so this seems an unlikely meaning of the title. Besides, I can’t imagine why saints would be “deathly.” That just seems like an oxymoron.
4) Continuation of my Google search then directed me to an Arthurian A-Z dictionary published on something called the “Mystical World Wide Web:” http://www.mystical-www.co.uk/arthuriana2z/h.htm#HAL. It’s a really fascinating explanation of “hallows,” but in brief:
a. In Arthurian legend and myth, a hallow is either a piece of royal regalia or a much-sought object of power/sacred vessel that is “quested” after like the holy grail.
b. Something called “The Hallows of Ireland” were brought to Ireland by something/one called the “Tuatha de Danaan” (Unlcear if this is a single person, a race of people or a group of some sort) and were guarded by four “Guardians of the Hollows.”
c. Beliefs later evolved/changed (again unclear how this happened) to say that the hallows brought by the Tuatha were four sacred treasures from the “otherworld:” the shining spear of Lugh (which provides victory in any fight), the stone of Fal (on which kings were crowned), the sword of Nuada (with which it is impossible to avoid being struck and wounded), and the cauldron of Dagda (plenty). In later traditions, these evolved into: the pole of combat, the sword of light, the cauldron of cure and the sword of destiny. In even later traditions, these have developed into the four symbols of magical elements: the sword, the spear, the cup and the pentacle—in other words, the four suits of Tarot cards.
Conclusion: My, my, a hallow sounds quite a bit like a horcrux. JKR knows her Arthurian mythology and stole the term as a loose synonym for “horcrux” in order to make the title more mysterious.
Better conclusion: JKR not only took the term from Arthurian mythology, but plans to parallel other aspects of book seven to the Arthurian legend of “hallows.” Notice that certain aspects of that story fit nicely with aspects of the HP books. There are four guardians of the hollows just like the four founders of Hogwarts. Dumbledore made it clear in book six that Voldemort would have looked for magical objects of power, such as objects that belonged to these founders. We’ve also seen a sword (one of the two known existing relics to belong to Godric Gryffindor) and a cup (a relic of Helga Hufflepuff presumably stolen by Voldemort and used as a horcrux). OK, so it’s not a perfect match—there’s on sacred locket in Arhurian mythology, and I’m not sure I know the difference between a sword and a spear, but you must admit there are parallels. Furthermore, I feel like she could work in the part about hallows being associated with “magical elements”--- this is, after all, a world of magic, and don’t they use some form of Tarot card in divination?

Comments
Ooo thanks for going researching. I was wondering that a bit myself, because there's a place tucked deep away in my brain that kind of knew what it meant, but I still wasn't sure. :)
I really want to make myself not read Book 7 all in one day (like I have the rest of them) but I'm not sure I've got that much will-power.
and interesting parallel, between the arthurian quest for the holy grail and harry's for the horcruxes...
Very interesting stuff about the Arthurian legends. I haven't read those in so long that I'd forgotten a lot of that stuff!