(Hemisphaerium, compounded of νεύρον: half, and σφαῖρα: sphere, in geometry, is one half of a globe or sphere, when divided into two by a plane passing through its centre.

in astronomy, is particularly used for one half of the mundane sphere.

The equator divides the sphere into two equal parts, called the northern and southern hemispheres. The horizon also divides the sphere into two parts, called the upper and the lower hemispheres.