Source : Webster's Revised Unabridged Dictionary (1913)

   {Silky, or Silk-bark}, {oak}, an Australian tree ({Grevillea
      robusta}).

   {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} (Zo["o]l.), 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}. (Zo["o]l.) 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.]

Source : WordNet®

oak apple
     n : oak gall caused by larvae of a cynipid wasp