  In differential-form representation, the Maxwell equations are represented by
simple differential relations between the electromagnetic two-forms and source
three-forms while the electromagnetic medium is defined through a constitutive
relation between the two-forms. The simplest of such relations expresses the
electromagnetic two-forms as scalar multiples of one another. Because of its
strange properties, the corresponding medium has been considered as
nonphysical. In this study such a medium is interpreted in terms of the
classical Gibbsian vectors as a bi-isotropic medium with infinite values for
its four medium parameters. It is shown that the medium is a generalization of
both PEC (perfect electric conductor) and PMC (perfect magnetic conductor)
media, with similar properties. This is why the medium is labeled as PEMC
(perfect electromagnetic conductor). Defining a certain class of duality
transformations, PEMC medium can be transformed to PEC or PMC media. As an
application, plane-wave reflection from a planar interface of air and PEMC
medium is studied. It is shown that, in general, the reflected wave has a
cross-polarized component, which is a manifestly nonreciprocal effect.
