The Collaborative International Dictionary of English v.0.48:

Standpipe \Stand"pipe`\ (-p[imac]p`), n.
   1. (Engin.) A vertical pipe, open at the top, between a
      hydrant and a reservoir, to equalize the flow of water;
      also, a large vertical pipe, near a pumping engine, into
      which water is forced up, so as to give it sufficient head
      to rise to the required level at a distance.
      [1913 Webster]

   2. (Steam Boiler) A supply pipe of sufficient elevation to
      enable the water to flow into the boiler, notwithstanding
      the pressure of the steam. --Knight.
      [1913 Webster]

WordNet (r) 3.0 (2006):

standpipe
    n 1: a vertical pipe