Perpendicular bisector. A perpendicular bisector, this will be shown by the green line, will label one side A, the other side B, and the intersecting point C. Again, this is a perpendicular bisector. This line will intersect a line segment at 90 degrees.
So angle OCA and OCB will be a 90 degree angle, which will divide the line segment into two equal parts. So AC will be one part. and CB will be the other equal part.