In Chapter 2 of Reference [1], one of the footnotes has a definition of a radian. For a unit circle with a radius of one unit of length called $u$, one radian is the angle corresponding to an arc-length of $u$. In degrees, one radian is approximately 57 degrees, which is perhaps easier to visualize. A diagram of one radian is included here for reference. References [1] Konrad Knopp. Theory and Application of Infinite Series. Dover Publications. 1990.