\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}.