From Stephanie Lott, City of Austin Watershed Protection:

"My understanding is that the spillway is where we want the water to flow if it gets high enough to overtop the dam.

"The channel or passageway around or over a dam through which excess water is released or "spilled" past the dam without going through the turbines. A spillway is a safety valve for a dam and, as such, must be capable of discharging major floods without damaging the dam, while maintaining the reservoir level below some predetermined maximum level."
(http://www.streamnet.org/pub-ed/ff/Glossary/glossarydam.html)

Or

“A structure over or through which excess or flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway, if the elevation of the spillway crest is the only control, it is an uncontrolled spillway.”
(http://www.srh.noaa.gov/wgrfc/resources/glossary/s.html)"