What is the original position of the radius vector called?

The original position of the radius vector is referred to as the initial side. In the context of circles and angles, the radius vector is typically represented as a line drawn from the center of the circle to a point on its circumference. The initial side denotes the position of this radius vector before any rotation or angle measurement has occurred.

When measuring angles in standard position, the initial side is aligned along the positive x-axis. From this position, the angle is formed by rotating the radius vector counterclockwise (or clockwise) around the origin. This concept is fundamental in trigonometry and helps in understanding angular measurements and their applications.

The other options pertain to different aspects of geometry related to angles. For instance, the terminal side refers to the position of the radius vector after it has been rotated through a specified angle. The reference angle is the acute angle formed between the terminal side of an angle and the x-axis. Magnitude, while important in various contexts, specifically refers to the length of a vector rather than its position. Thus, the term "initial side" specifically captures the concept of the starting position of the radius vector before any movement, solidifying its correctness as the answer.