One radian is when the size of the intercepted arc is equal to the radius of the circle.
This supplement will help explain this definition.
Move the terminal point until the green arc is the same length as the radius.
Change the radius and do it again.
Click to check how many degrees are in one radian.

How many degrees are in one radian?
How many radians are there in a unit circle? Radius = 1
Change the radius to 2 and find how many radians are in this circle.
How many radians are in 90 degrees?