A segment bisector is an object that passes through the midpoint of a line segment, bisecting it or dividing it into two segments of equal length. The object can be a point, a segment, a line, a ray, or a plane.

If $M$ is the midpoint of $\overline{AB},$ then it is a segment bisector of $\overline{AB}.$ Furthermore, since line $\ell$ passes through $M,$ it is also a segment bisector of $\overline{AB}.$