Merriam-Webster Collegiate Dictionary - hemisphere

 noun  Etymology: Middle English hemispere, from Latin hemisphaerium, from Greek hemisphairion, from hemi- + sphairion, diminutive of sphaira sphere  Date: 14th century  1.  a. a half of the celestial sphere as divided into two halves by the horizon, the celestial equator, or the ecliptic  b. half of a spherical or roughly spherical body (as a planet); specifically the northern or southern half of the earth as divided by the equator or the eastern or western half as divided by a meridian  c. the inhabitants of a terrestrial ~  2. realm, province  3. one of two half spheres formed by a plane through the sphere's center  4. a map or projection of a celestial or terrestrial ~  5. cerebral ~  • hemispheric or hemispherical adjective