A line which
is drawn at right angles to a line segment AB and divides it in
half is called its perpendicular bisector. Any point P on this line
is the same distance from A as it is from B.

The perpendicular bisector
can be constructed by using compasses to draw circular arcs of equal
radius centred on A and B, and joining the two points where they
intersect by a straight line.

The three perpendicular bisectors of the sides of a triangle
meet at the same point. This is the center of the triangle's
circumcircle, the circle through its vertices.