[a. L. type *versor, f. vers-, vertĕre to turn.]
† 1. The needle of a compass. Obs. rare.
1640. G. Watts, trans. Bacon’s Adv. Learn., V. ii. 229. So the versor of a Mariners needle applies it selfe to the Poles of the world.
2. Math. In quaternions, an operator that changes the direction of a vector without altering its length.
a. 1865. Sir W. R. Hamilton, Elem. Quaternions, II. i. (1866), 133. We shall now say that every Radial Quotient is a Versor. A Versor has thus, in general, a plane, an axis, and an angle.
1886. W. S. Aldis, Solid Geom. (ed. 4), xiv. 235. If the two vectors OA and OB be of equal length the change of one into the other is merely an operation of rotation. In this case the quaternion is called a versor.
