Electromagnetic Nonreciprocity

$Nphantom{rule{0}{0ex}}ophantom{rule{0}{0ex}}nphantom{rule{0}{0ex}}rphantom{rule{0}{0ex}}ephantom{rule{0}{0ex}}cphantom{rule{0}{0ex}}iphantom{rule{0}{0ex}}pphantom{rule{0}{0ex}}rphantom{rule{0}{0ex}}ophantom{rule{0}{0ex}}cphantom{rule{0}{0ex}}iphantom{rule{0}{0ex}}tphantom{rule{0}{0ex}}y$ has emerged as a fundamental concept in modern science and technology. Recent advances in magnet-free nonreciprocal systems have spurred a resurgence of interest in this subject, but have also led to questions and confusion. The authors review nonreciprocity from a global, first-principles perspective, with the aim of establishing a solid foundation for the topic, to resolve previous issues and promote forthcoming research in the field.