Abstract: Multiple global crises—including the pandemic, climate change, and Russia’s war on Ukraine—have recently linked together in ways that are significant in scope, devastating in effect, but poorly understood. A growing number of scholars and policymakers characterise the situation as a “poly-crisis.” Yet this neologism remains poorly defined. This paper sets out the concept with a substantive definition, highlights its value in comparison to related concepts, and provides a theoretical framework to explain the causal mechanisms currently entangling many of the world’s crises. In this framework, a global crisis arises when one or more fast-moving trigger events combines with slow-moving stresses to push a global system out of its established equilibrium and into a volatile and harmful state of disequilibrium. The authors then identify three causal pathways—common stresses, domino effects, and inter-systemic feedbacks—that can connect multiple global systems to produce synchronised crises.