WordNet (r) 3.0 (2006):

burr oak
    n 1: medium to large deciduous oak of central and eastern North
         America with ovoid acorns deeply immersed in large fringed
         cups; yields tough close-grained wood [syn: bur oak,
         burr oak, mossy-cup oak, mossycup oak, Quercus
         macrocarpa]