A formal definition of the radian is: One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. – Wikipedia A better (simpler) definition at Math Open Reference is: One radian is the angle made at the center of a circle by an arc whose length is equal to the radius of the circle. What they are saying is: If you have a circle and pick a point on its edge (perimeter / circumference) and travel along that edge a distance equal to the length of the radius (the distance from the center of the circle to its edge), then the angle, at the center of the circle, between those two points is equal to one radian. One radian is just … Read entire article »