Radians

Radians are a unit of measurement used to quantify angles in geometry and trigonometry. One radian is defined as the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. In other words, if you were to wrap a string around the circumference of a circle, and that string is the same length as the radius of the circle, the angle between the two ends of the string would be one radian. 

Radians are commonly used in mathematical and scientific calculations involving angles because they have certain advantages over degrees. In radians, the relationship between the length of an arc, the radius, and the angle is straightforward: the angle in radians is equal to the arc length divided by the radius. This makes radians a more natural choice for calculus and trigonometry, as it simplifies many mathematical formulas and calculations involving angles. There are 2π radians in a full circle, which is equivalent to 360 degrees.