The Collaborative International Dictionary of English v.0.48:

Pylon \Py"lon\, n. [NL., fr. Gr. ? a gateway.]
   (a) A low tower, having a truncated pyramidal form, and
       flanking an ancient Egyptian gateway.
       [1913 Webster]

             Massive pylons adorned with obelisks in front. --J.
                                                  W. Draper.
       [1913 Webster]
   (b) An Egyptian gateway to a large building (with or without
       flanking towers).
       [1913 Webster]

   2. A tower, commonly of steelwork, for supporting either end
      of a wire, as for a telegraph line, over a long span.
      [Webster 1913 Suppl.]

   3. (Aeronautics)
      (a) Formerly, a starting derrick (the use of which is now
          abandoned) for an aeroplane.
      (b) A post, tower, or the like, as on an aerodrome, or
          flying ground, serving to bound or mark a prescribed
          course of flight.
          [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

pylon
    n 1: a tower for guiding pilots or marking the turning point in
         a race
    2: a large vertical steel tower supporting high-tension power
       lines; "power pylons are a favorite target for terrorists"
       [syn: pylon, power pylon]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "pylon":
   French door, antenna tower, archway, back door, barbican, barway,
   beacon, beam, belfry, bell tower, bulkhead, campanile,
   carriage entrance, cellar door, cellarway, colossus, column,
   cupola, derrick, dome, door, doorjamb, doorpost, doorway,
   fan marker, fire tower, flare path, front door, gate, gatepost,
   gateway, hatch, hatchway, lantern, lighthouse, lintel, marker,
   martello, martello tower, mast, minaret, monument, obelisk,
   observation tower, pagoda, pilaster, pillar, pinnacle, pole, porch,
   portal, porte cochere, postern, propylaeum, pyramid, racon,
   radar beacon, radio beacon, scuttle, shaft, side door, skyscraper,
   spire, standpipe, steeple, stile, storm door, stupa,
   television mast, threshold, tollgate, tope, tour, tower, trap,
   trap door, turnpike, turnstile, turret, water tower, wind cone,
   wind indicator, wind sock, windmill tower