The Collaborative International Dictionary of English v.0.48:

fire hydrant \fire hydrant\ n.
   an upright hydrant that can supply large volumes of water to
   use in fighting a fire. They are commonly placed at intervals
   at the street edge of a sidewalk, spaced for convenience in
   suppressing fires in towns.

   Syn: fireplug, plug.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

fire hydrant
    n 1: an upright hydrant for drawing water to use in fighting a
         fire [syn: fireplug, fire hydrant, plug]