WordNet (r) 3.0 (2006):

jack oak
    n 1: a common scrubby deciduous tree of central and southeastern
         United States having dark bark and broad three-lobed (club-
         shaped) leaves; tends to form dense thickets [syn:
         blackjack oak, blackjack, jack oak, Quercus
         marilandica]
    2: small to medium deciduous oak of east central North America;
       leaves have sharply pointed lobes [syn: jack oak, northern
       pin oak, Quercus ellipsoidalis]