In pre-calculus, the unit circle is sort of like unit streets, it’s the very small circle on a graph that encompasses the coordinates. It has a radius of hence the unit. The figure here shows all the measurements of the unit circle:
Calculus for Beginners and Artists is an online textbook that provides an overview of Calculus in clear, easy to understand language designed for the non-mathematician. Thank you MIT OpenCourseWare for awesome, free resources and classes!