A radian is a unit of angle, equal to an angle at the center of a circle whose arc is equal in length to the radius.In simpiler terms to explain it to someone else I would say it is the same thing as a degree just in a different form, it always includes pi. The unit circle includes radians, radians are the degree of the angle inside the unit circle, there are multiple of these units for example one is 3.14/6. which is a 30 degree angle in quadrant I. The formula of a circumfrance is 2(pi)r which is all the way around a circle, this relates to radians because one full rotation around a circle is 2(pi). They are equal when the radius is 1 in the unit cirlce which is basically always. A radian is a degree just in a different form, you can cross multiply to switch them, like (pi)/3 is the exact same thing as 30 degrees. I perfer degrees because I find it easier to use them instead of a fraction with pi in it. More mathimatically pure would be radians, when you use degrees you could always get a decimal while in radians it will always be correct and with the actual pi.