The Collaborative International Dictionary of English v.0.48:

Sylph \Sylph\, n. [F. sylphe, m., fr. Gr. ? a kind of grub,
   beetle, or moth; -- so called by Paracelsus.]
   1. An imaginary being inhabiting the air; a fairy.
      [1913 Webster]

   2. Fig.: A slender, graceful woman.
      [1913 Webster]

   3. (Zool.) Any one of several species of very brilliant South
      American humming birds, having a very long and
      deeply-forked tail; as, the blue-tailed sylph (Cynanthus
      cyanurus).
      [1913 Webster]

WordNet (r) 3.0 (2006):

sylph
    n 1: a slender graceful young woman
    2: an elemental being believed to inhabit the air

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "sylph":
   Ariel, Befind, Corrigan, Finnbeara, Mab, Oberon, Titania, banshee,
   brownie, cluricaune, dwarf, elemental, elemental spirit, elf,
   fairy, fairy queen, fay, gnome, goblin, gremlin, hob, imp, kobold,
   leprechaun, ouphe, peri, pixie, pooka, puca, pwca, salamander,
   sprite, sylphid, undine



The Devil's Dictionary (1881-1906):

SYLPH, n.  An immaterial but visible being that inhabited the air when
the air was an element and before it was fatally polluted with factory
smoke, sewer gas and similar products of civilization.  Sylphs were
allied to gnomes, nymphs and salamanders, which dwelt, respectively,
in earth, water and fire, all now insalubrious.  Sylphs, like fowls of
the air, were male and female, to no purpose, apparently, for if they
had progeny they must have nested in accessible places, none of the
chicks having ever been seen.