1. 1. Any of numerous monoecious deciduous or evergreen trees or shrubs of the genus Quercus, bearing acorns as fruit.
   2. The durable wood of any of these trees or shrubs.
   3. Something made of this wood.
2. Any of various similar trees or shrubs, such as the poison oak.
3. Any of various brown shades resembling the wood of an oak in color.

[Middle English ok, from Old English āc.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.