Book: Calculus by Thomas/Finney. 9th Edition
Radian is a unit of measurment. In calculus, angle is measured in radian. (Navigation and astronomy uses degrees for angles) – page 35
Radian measure $\theta$ is given by $\frac{s}{r}$ wher $s$ is arc length AB and $r$ is the radius.
Six basic trigonometric functions.
Some other functions.
For more: Calculus book by Thomas/Finney (9th Edition); Pages 35-50