A "bisector" in geometry can be either a segment bisector or an angle bisector.
A segment bisector is a segment, line, or ray that intersects the midpoint of the segment being bisected. The bisected segment is partitioned into 2 segments of equal length.
An angle bisector is a ray, segment, or line intersecting the vertex of the angle being bisected. The bisected angle is partitioned into 2 adjacent angles of equal measure.
Note that a bisector must be a segment, line, or ray. A midpoint, though it divides a segment in two, is simply called a midpoint, not a bisector.