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A Scalar which reverses sign under inversion is called a pseudoscalar.  The Scalar Triple Product 
See also Pseudotensor, Pseudovector, Scalar
References
Arfken, G.  ``Pseudotensors, Dual Tensors.''  §3.4 in
  Mathematical Methods for Physicists, 3rd ed.  Orlando, FL: Academic Press, pp. 128-137, 1985.