noun

1. half of a sphere, globe, or celestial body; specif.,
   1. any of the halves into which the celestial sphere is divided by either the celestial equator or the ecliptic
   2. any of the halves of the earth: the earth is divided by the equator into the N and S hemispheres and by a meridian into the E Hemisphere (containing Europe, Asia, Africa, and Australia) and the W Hemisphere (containing the Americas and Oceania)
   3. a model or map of any of these halves
2. the countries and peoples of any of the earth's hemispheres
3. an area of action, knowledge, etc.
4. either lateral half of the cerebrum or cerebellum

Etymology: ME hemisperie < L hemisphaerium < Gr hēmisphairion: see hemi- & sphere