An apothem is a line segment (or the length of the line segment) with one endpoint at the centre of a circle and the other endpoint on the midpoint of a chord of the circle.

For a regular polygon, the apothem is a line segment (or the length of the line segment) from the centre of the polygon to the midpoint of a side of the polygon.

Because of the "Chord Property", the apothem is perpendicular to the related chord (circle) or side (regular polygon).