What is $1$ radian? What is $1$ steradian?

(N/A) $1$ radian: The plane angle subtended at the center of a circle by an arc equal in length to the radius of the circle is defined as $1$ radian.
$1$ steradian: The solid angle subtended at the center of a sphere by a surface area equal to the square of the radius of the sphere is defined as $1$ steradian.