The Collaborative International Dictionary of English v.0.48:

Rune \Rune\ (r[udd]n), n. [AS. r[=u]n a rune, a secret, a
   mystery; akin to Icel. r[=u]n, OHG. & Goth. r[=u]na a secret,
   secret colloquy, G. & Dan. rune rune, and probably to Gr.
   'ereyna^n to search for. Cf. Roun to whisper.]
   1. A letter, or character, belonging to the written language
      of the ancient Norsemen, or Scandinavians; in a wider
      sense, applied to the letters of the ancient nations of
      Northern Europe in general.
      [1913 Webster]

   Note: The Norsemen had a peculiar alphabet, consisting of
         sixteen letters, or characters, called runes, the
         origin of which is lost in the remotest antiquity. The
         signification of the word rune (mystery) seems to
         allude to the fact that originally only a few were
         acquainted with the use of these marks, and that they
         were mostly applied to secret tricks, witchcrafts and
         enchantments. But the runes were also used in
         communication by writing.
         [1913 Webster]

   2. pl. Old Norse poetry expressed in runes.
      [1913 Webster]

            Runes were upon his tongue,
            As on the warrior's sword.            --Longfellow.
      [1913 Webster]

   Rune stone, a stone bearing a runic inscription.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rune
    n 1: any character from an ancient Germanic alphabet used in
         Scandinavia from the 3rd century to the Middle Ages; "each
         rune had its own magical significance" [syn: rune, runic
         letter]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "rune":
   alchemy, arrowhead, bewitchery, character, charm, conjuration,
   cuneiform, demotic character, determinative, divination,
   enchantment, fetishism, glamour, gramarye, grammalogue,
   hieratic symbol, hieroglyph, hieroglyphic, hieroglyphics, hiragana,
   hoodoo, ideogram, ideograph, incantation, juju, jujuism, kana,
   katakana, logogram, logograph, magic, natural magic, necromancy,
   obeah, ogham, phonetic, phonetic symbol, pictogram, pictograph,
   poesy, poetry, radical, rhyme, shamanism, shorthand, sorcery,
   sortilege, spell, spellbinding, spellcasting, sympathetic magic,
   thaumaturgia, thaumaturgics, thaumaturgism, thaumaturgy, theurgy,
   vampirism, verse, voodoo, voodooism, wanga, wedge, white magic,
   witchcraft, witchery, witchwork, wizardry, word letter