In geometry, the apothem of a regular polygon is a line drawn from the center of the polygon perpendicular to one of its sides. In other words, a line drawn from the center of a polygon to the midpoint of its sides. Geometry (from the Greek words Ge = earth and metro = measure) is the branch of mathematics first introduced by Theaetetus dealing with spatial relationships. ... A polygon (from the Greek poly, for many, and gonos, for angle) is a closed planar path composed of a finite number of sequential straight line segments. ...