/ ˈhemɪsfɪə(r); NAmE -sfɪr/ noun
1.
one half of the earth, especially the half above or below the equator :
the northern / southern hemisphere
2.
either half of the brain :
the left / right cerebral hemisphere
3.
one half of a sphere (= a round solid object)
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WORD ORIGIN
late Middle English (in the sense half the celestial sphere, the sky ): from Old French emisphere , via Latin from Greek hēmisphairion , from hēmi- half + sphaira sphere.