A circle is a figure in which each point around its perimeter is an equal distance from the center. The **radius** of a circle is the distance between the center and any point along its perimeter (AC, CB, CD). A **chord** is a line segment that connects any two points along its perimeter (AB, AD, BD). The **diameter** of a circle is the length of a chord that passes through the center of the circle (AB) and equals twice the circle's radius (2r).

Line segment AB is this circle's __________.