Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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