Noun
an angle formed by two straight lines (in the same plane)
Source: WordNetDefinition Radian describes the plane angle subtended by a circular arc as the length of the arc divided by the radius of the arc. Source: Internet
Therefore one steradian corresponds to the plane (i.e. radian) angle of the cross-section of a simple cone subtending the plane angle 2θ, with θ given by: : This angle corresponds to the plane aperture angle of 2θ ≈ 1.144 rad or 65.54°. Source: Internet