Source : Webster's Revised Unabridged Dictionary (1913)

Versor \Ver"sor\, n. [NL., fr. L. vertere, versus, to turn. See
   {Version}.] (Geom.)
   The turning factor of a quaternion.

   Note: The change of one vector into another is considered in
         quaternions as made up of two operations; 1st, the
         rotation of the first vector so that it shall be
         parallel to the second; 2d, the change of length so
         that the first vector shall be equal to the second.
         That which expresses in amount and kind the first
         operation is a versor, and is denoted geometrically by
         a line at right angles to the plane in which the
         rotation takes place, the length of this line being
         proportioned to the amount of rotation. That which
         expresses the second operation is a tensor. The product
         of the versor and tensor expresses the total operation,
         and is called a quaternion. See {Quaternion}.

   {Quadrantal versor}. See under {Quadrantal}.