The green flash phenomenon seen at sunset.
green flash definition by Webster's New World
Webster\'s New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a sudden, bright flash of greenish light sometimes visible near the horizon at sunset or sunrise on a clear day, caused by the atmospheric refraction of sunlight
green flash definition by Wiktionary
(plural green flashes)