Bisection

In geometry, bisection is the division of something into two equal parts. This is usually done by a line, which we will call the "bisector". The most known types are the "segment bisector" (a line that passes through the middle of a given segment) and the "angle bisector" (a line that passes through the apex of an angle, that divides it into two equal angles. 
In three-dimensional space, bisection is usually done by a plane, which is also called the bisector or bisecting plane.