oak apple

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A round spongy gall on an oak leaf or twig, caused by the larvae of certain gall wasps.

from The Century Dictionary.

• noun In Australia, a cone of any one of several species of Casuarina, or she-oak, especially C. stricta. The young cones and foliage have an acid taste when chewed, and afford great relief in cases of thirst. See she-oak and Casuarina.
• noun An oak-gall. See gal.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Oak gall.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun oak gall caused by larvae of a cynipid wasp