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]