The Collaborative International Dictionary of English v.0.48:

Oak \Oak\ ([=o]k), n. [OE. oke, ok, ak, AS. [=a]c; akin to D.
   eik, G. eiche, OHG. eih, Icel. eik, Sw. ek, Dan. eeg.]
   [1913 Webster]
   1. (Bot.) Any tree or shrub of the genus Quercus. The oaks
      have alternate leaves, often variously lobed, and
      staminate flowers in catkins. The fruit is a smooth nut,
      called an acorn, which is more or less inclosed in a
      scaly involucre called the cup or cupule. There are now
      recognized about three hundred species, of which nearly
      fifty occur in the United States, the rest in Europe,
      Asia, and the other parts of North America, a very few
      barely reaching the northern parts of South America and
      Africa. Many of the oaks form forest trees of grand
      proportions and live many centuries. The wood is usually
      hard and tough, and provided with conspicuous medullary
      rays, forming the silver grain.
      [1913 Webster]

   2. The strong wood or timber of the oak.
      [1913 Webster]

   Note: Among the true oaks in America are:

   Barren oak, or

   Black-jack, Quercus nigra.

   Basket oak, Quercus Michauxii.

   Black oak, Quercus tinctoria; -- called also yellow oak
      or quercitron oak.

   Bur oak (see under Bur.), Quercus macrocarpa; -- called
      also over-cup or mossy-cup oak.

   Chestnut oak, Quercus Prinus and Quercus densiflora.

   Chinquapin oak (see under Chinquapin), Quercus
      prinoides.

   Coast live oak, Quercus agrifolia, of California; -- also
      called enceno.

   Live oak (see under Live), Quercus virens, the best of
      all for shipbuilding; also, Quercus Chrysolepis, of
      California.

   Pin oak. Same as Swamp oak.

   Post oak, Quercus obtusifolia.

   Red oak, Quercus rubra.

   Scarlet oak, Quercus coccinea.

   Scrub oak, Quercus ilicifolia, Quercus undulata, etc.


   Shingle oak, Quercus imbricaria.

   Spanish oak, Quercus falcata.

   Swamp Spanish oak, or

   Pin oak, Quercus palustris.

   Swamp white oak, Quercus bicolor.

   Water oak, Quercus aquatica.

   Water white oak, Quercus lyrata.

   Willow oak, Quercus Phellos.
      [1913 Webster] Among the true oaks in Europe are:

   Bitter oak, or

   Turkey oak, Quercus Cerris (see Cerris).

   Cork oak, Quercus Suber.

   English white oak, Quercus Robur.

   Evergreen oak,

   Holly oak, or

   Holm oak, Quercus Ilex.

   Kermes oak, Quercus coccifera.

   Nutgall oak, Quercus infectoria.
      [1913 Webster]

   Note: Among plants called oak, but not of the genus
         Quercus, are:

   African oak, a valuable timber tree (Oldfieldia
      Africana).

   Australian oak or She oak, any tree of the genus
      Casuarina (see Casuarina).

   Indian oak, the teak tree (see Teak).

   Jerusalem oak. See under Jerusalem.

   New Zealand oak, a sapindaceous tree (Alectryon
      excelsum).

   Poison oak, a shrub once not distinguished from poison ivy,
      but now restricted to Rhus toxicodendron or Rhus
      diversiloba.

   Silky oak or Silk-bark oak, an Australian tree
      (Grevillea robusta).
      [1913 Webster]

   Green oak, oak wood colored green by the growth of the
      mycelium of certain fungi.

   Oak apple, a large, smooth, round gall produced on the
      leaves of the American red oak by a gallfly (Cynips
      confluens). It is green and pulpy when young.

   Oak beauty (Zool.), a British geometrid moth (Biston
      prodromaria) whose larva feeds on the oak.

   Oak gall, a gall found on the oak. See 2d Gall.

   Oak leather (Bot.), the mycelium of a fungus which forms
      leatherlike patches in the fissures of oak wood.

   Oak pruner. (Zool.) See Pruner, the insect.

   Oak spangle, a kind of gall produced on the oak by the
      insect Diplolepis lenticularis.

   Oak wart, a wartlike gall on the twigs of an oak.

   The Oaks, one of the three great annual English horse races
      (the Derby and St. Leger being the others). It was
      instituted in 1779 by the Earl of Derby, and so called
      from his estate.

   To sport one's oak, to be "not at home to visitors,"
      signified by closing the outer (oaken) door of one's
      rooms. [Cant, Eng. Univ.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

post oak
    n 1: small deciduous tree of eastern and central United States
         having dark green lyrate pinnatifid leaves and tough
         moisture-resistant wood used especially for fence posts
         [syn: post oak, box white oak, brash oak, iron oak,
         Quercus stellata]