The Collaborative International Dictionary of English v.0.48:

Hydrant \Hy"drant\, n. [Gr. ? to irrigate, fr. "y`dwr water. See
   Hydra.]
   A discharge pipe with a valve and spout at which water may be
   drawn from the mains of waterworks; a water plug.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hydrant
    n 1: a faucet for drawing water from a pipe or cask [syn: water
         faucet, water tap, tap, hydrant]
    2: a discharge pipe with a valve and spout at which water may be
       drawn from the mains of waterworks

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "hydrant":
   Foamite, acid, automatic sprinkler, carbon tet,
   carbon tetrachloride, carbon-dioxide foam, cock, deck gun,
   deluge set, extinguisher, fire apparatus, fire engine, fire hose,
   fire hydrant, fireplug, foam, foam extinguisher, gate,
   hook-and-ladder, ladder pipe, petcock, plug, pumper, snorkel, soda,
   spigot, sprinkler, sprinkler head, sprinkler system, stopcock,
   super-pumper, tap, valve, water, water cannon, water plug,
   wet blanket