An acorn cluster of the bur oak tree.
bur oak 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 North American white oak (Quercus macrocarpa) having large acorns with fringed cups; mossy-cup oak
bur oak definition by American Heritage Dictionary
An oak tree (Quercus macrocarpa) of eastern North America, having pinnately lobed leaves, acorns enclosed within a deep fringed cup, and hard durable wood.