Electromagnetic Nonreciprocity

$N{{0}{0ex}}o{{0}{0ex}}n{{0}{0ex}}r{{0}{0ex}}e{{0}{0ex}}c{{0}{0ex}}i{{0}{0ex}}p{{0}{0ex}}r{{0}{0ex}}o{{0}{0ex}}c{{0}{0ex}}i{{0}{0ex}}t{{0}{0ex}}y$ 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.