1. 1. A half of a sphere bounded by a great circle.
   2. A half of a symmetrical, approximately spherical object as divided by a plane of symmetry.
2. Either half of the celestial sphere as divided by the ecliptic, the celestial equator, or the horizon.
3. Either the northern or southern half of the earth as divided by the equator or the eastern or western half as divided by a meridian.
4. Anatomy. Either of the lateral halves of the cerebrum; a cerebral hemisphere.

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.