What is a heimisphere?

Answer 1

a hemisphere is a part or section of the earth(like the northern and southern halves of the earth, those are hemispheres).

Answer 2

A hemisphere is a half of a sphere or a celestial body, divided by an imaginary plane. It is commonly used to refer to the halves of Earth divided by the equator into the Northern Hemisphere and Southern Hemisphere or by the Prime Meridian into the Eastern Hemisphere and Western Hemisphere.