Angle bisector

An angle bisector starts at the angle’s vertex and splits the angle exactly in half.

Every point on an angle bisector is equidistant from the two arms of the angle, where distance from a point to a line is measured perpendicularly. Conversely, a point inside an angle that is equidistant from its arms lies on the angle bisector. This is why an angle bisector can also represent a locus, although the construction here is simply the required ruler-and-compass method.