bisection -
A division into two parts, especially into two equal parts.
Wikipedia
Bisection is the division of something into two equal or congruent parts, usually by a line, which is then called a bisector. The most often considered types of bisectors are the segment bisector (a line that passes through the midpoint 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, also called the bisector or bisecting plane.
