oak-leaf cluster n (Military) US an insignia consisting of oak leaves and acorns awarded to holders of certain military decorations to indicate a further award of the same decoration Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014