The Collaborative International Dictionary of English v.0.48:

Spillway \Spill"way`\, n.
   A sluiceway or passage for superfluous water in a reservoir,
   to prevent too great pressure on the dam.
   [1913 Webster]

WordNet (r) 3.0 (2006):

spillway
    n 1: a channel that carries excess water over or around a dam or
         other obstruction [syn: spillway, spill, wasteweir]