One radian is the central angle intercepting an arc equal in length to the:

1. The definition ties an arc length to a length in the circle. 2. That length is the distance from centre to edge. 3. One radian is the measure of a central angle that intercepts an arc equal in length to the radius. 4. So the answer is the radius. _Source: OpenStax 'Algebra and Trigonometry 2e', Ch 7 'The Unit Circle: Sine and Cosine Functions'_