A single four-leaf clover.
four-leaf clover 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 leaf of clover with four leaflets on one leafstalk, popularly believed to bring good luck to the finder
four-leaf clover definition by American Heritage Dictionary
A clover leaf having four leaflets instead of the normal three, considered to be an omen of good luck.