oak-leaf cluster 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 small, bronze cluster of oak leaves and acorns awarded to the holder of a U.S. Army or Air Force decoration for each new award of that decoration: a silver cluster equals five bronze clusters
oak-leaf cluster definition by American Heritage Dictionary
A decoration of bronze or silver oak leaves and acorns given to holders of various U.S. military medals in recognition of acts entitling them to another award of the same medal.