Earlier, the linear magnetoelectric effect of chromium sesquioxide Cr2O3 has been determined experimentally as a function of temperature. One measures the electric field-induced magnetization on Cr2O3 crystals or the magnetic field-induced polarization. From the magnetoelectric moduli of Cr2O3 we extract a 4-dimensional relativistic invariant pseudoscalar View the MathML source. It is temperature dependent and of the order of not, vert, similar10−4Y0, with Y0 as vacuum admittance. We show that the new pseudoscalar View the MathML source is odd under parity transformation and odd under time inversion. Moreover, View the MathML source is for Cr2O3 what Tellegen's gyrator is for two port theory, the axion field for axion electrodynamics, and the PEMC (perfect electromagnetic conductor) for electrical engineering.