In the diagram below, AB is the chord of a circle with centre O. OM is perpendicular to AB (meet at a right angle). Look at triangles OAM and OBM. The hypotenuses (OA and OB) are the same ...

they are said to be perpendicular. A line drawn to cut another line or angle in half is a bisector. The path of a point that moves according to a certain rule is a locus. The plural of locus is loci.