The perpendicular bisector of a side of a triangle a line perpendicular to the side and passing through its midpoint.

The three perpendicular bisectors of the sides of a triangle meet in a single point, called the circumcenter.

The circumcenter is equidistant from the vertices of the triangle. (See circumcenter theorem.)
The angle bisector of an angle of a triangle is a straight line that divides the angle into two congruent angles.

The three angle bisectors of the angles of a triangle meet in a single point, called the incenter.
