Do you know ghouls from goblins and ghosts?
"one of a race of powerful supernatural beings in Germanic folklore," Old English elf (Mercian, Kentish), ælf (Northumbrian), ylfe (plural, West Saxon), from Proto-Germanic *albiz (cf. Old Saxon alf, Old Norse alfr, German alp "evil spirit, goblin, incubus"), origin unknown, possibly from PIE *albho- "white." Used figuratively for "mischievous person" from 1550s.
In addition to elf/ælf (masc.), Old English had parallel form *elfen (fem.), the plural of which was *elfenna, -elfen, from Proto-Germanic *albinjo-. Both words survived into Middle English and were active there, the former as elf (with the vowel of the plural), plural elves, the latter as elven, West Midlands dialect alven (plural elvene).
The Germanic elf originally was dwarfish and malicious (cf. Old English ælfadl "nightmare," ælfsogoða "hiccup," thought to be caused by elves); in the Middle Ages they were confused to some degree with faeries; the more noble version begins with Spenser. Nonetheless a popular component in Anglo-Saxon names, many of which survive as modern given names and surnames, cf. Ælfræd "Elf-counsel" (Alfred), Ælfwine "Elf-friend" (Alvin), Ælfric "Elf-ruler" (Eldridge), also women's names such as Ælfflæd "Elf-beauty." Elf Lock hair tangled, especially by Queen Mab, "which it was not fortunate to disentangle" [according to Robert Nares' glossary of Shakespeare] is from 1592.
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in Germanic folklore, originally, a spirit of any kind, later specialized into a diminutive creature, usually in tiny human form. In the Prose, or Younger, Edda, elves were classified as light elves (who were fair) and dark elves (who were darker than pitch); these classifications are roughly equivalent to the Scottish seelie court and unseelie court. The notable characteristics of elves were mischief and volatility. They were believed at various times and in various regions to cause diseases in humans and cattle, to sit upon the breast of a sleeper and give him bad dreams (the German word for nightmare is Alpdrucken, or "elf-pressure"), and to steal human children and substitute changelings (deformed or weak elf or fairy children). In the British Isles, flint implements called elf-bolts, elf-arrows, or elf-shot (which are now known to be prehistoric tools used by the aboriginal Irish and the early Scots) were believed to be the weapons with which elves injured cattle. Elves occasionally also were benevolent and helpful. The second edition of Encyclopaedia Britannica, which was published in 1777-84, calls the word elf obsolete but reports that belief in such creatures "still subsists in many parts of our own country. . . In the Highlands of Scotland, new-born children are watched till the christening is over, lest they should be stolen or changed by some of these phantastical existences." In time, elves came to be indistinct from fairies, though both older classics-such as Johann Wolfgang von Goethe's poem "Der Erlkonig" ("The Elf King")-and such modern classics as J.R.R. Tolkien's Lord of the Rings (1954-55) still treat elves as a distinct type.