WordNet (r) 3.0 (2006):

floodgate
    n 1: something that restrains a flood or outpouring; "suspension
         of surveillance opened the floodgates to illegal
         immigrants"
    2: regulator consisting of a valve or gate that controls the
       rate of water flow through a sluice [syn: sluicegate,
       sluice valve, floodgate, penstock, head gate, water
       gate]