 The term originated with William Kingdon Clifford, in showing that the quaternion algebra is just a special case of Hermann Grassmann's "theory of extension" (Ausdehnungslehre).Hestenes defined a rotor to be any element formula_2 of a geometric algebra that can be written as the product of an even number of unit vectors and satisfies formula_3, where formula_4 is the "reverse" of formula_2—that is, the product of the same vectors, but in reverse order.Reflections along a vector in geometric algebra may be represented as (minus) sandwiching a multivector "M" between a non-null vector "v" perpendicular to the hyperplane of reflection and that vector's inverse "v"−1