A radius of a circle is a line segment from the center of the circle to a point on the circle. The line segment $\displaystyle{\overline{{{O}{D}}}}\usepackage{cmbright}\Huge\$ is a radius because point O is the center of the circle and point D is on the circle. A segment can be named with either end point named first. $\displaystyle{\overline{{{D}{O}}}}\usepackage{cmbright}\Huge\$ and $\displaystyle{\overline{{{O}{D}}}}\usepackage{cmbright}\Huge\$ are the same line segment.