What mathematicians often call Clifford algebra is calledgeometric algebra if the focus is on the geometric meaning of the algebraic expressions and operators. Geometric algebra is a mathematical framework to easily describe geometric concepts and oper- ations. It allows us to develop algorithms fast and in an intuitive way. It is based on the work of Hermann Grassmann [A2] and his vision of a general mathematical language for geometry. William Clifford combined Grassmann's exterior algebra and Hamilton's quaternions [Clifford 1882a, 1882b]. Pioneering work has been done by David Hestenes, who first applied geometric algebra to problems in mechanics and physics [Hestenes and Sobczyk 1984; Hestenes 1985].His work culminated some years ago in the invention of conformal geometric algebra [Hestenes 2001].