  This paper aims at providing a global perspective on electromagnetic
nonreciprocity and clarifying confusions that arose in the recent developments
of the field. It provides a general definition of nonreciprocity and classifies
nonreciprocal systems according to their linear time-invariant (LTI), linear
time-variant (LTV) or nonlinear nonreciprocal natures. The theory of nonlinear
systems is established on the foundation of the concepts of time reversal,
time-reversal symmetry, time-reversal symmetry breaking and related Onsager-
Casimir relations. Special attention is given to LTI systems, as the most
common nonreciprocal systems, for which a generalized form of the Lorentz
reciprocity theorem is derived. The delicate issue of loss in nonreciprocal
systems is demystified and the so-called thermodynamics paradox is resolved
from energy conservation considerations. The fundamental characteristics and
applications of LTI, LTV and nonlinear nonreciprocal systems are overviewed
with the help of pedagogical examples. Finally, asymmetric structures with
fallacious nonreciprocal appearances are debunked.
