n (astronomy), (astrology) Half of the celestial sphere, as divided by either the ecliptic or the celestial equator (defdate|from 14th c.).
n (geography) Half of the Earth, such as the Northern Hemisphere, Southern Hemisphere, Western Hemisphere or Eastern Hemisphere, Land Hemisphere, Water Hemisphere etc. (defdate|1550s).
n (geometry) Any half-sphere, formed by a plane intersecting the center of a sphere. (defdate|1580s).