Recent legislative efforts across multiple jurisdictions aim at regulating existential risk, a new category of extreme risks that pose a greater threat to humanity’s future than any previous risk. This article investigates the ordinary meaning of legally relevant concepts in the existential risk literature. Four experiments (n=6,814) reveal that the ordinary meaning of “existential risk”: (a) like the technical meaning, is narrower than other related terms, such as “global catastrophic risk” and “extreme risk”; (b) is mostly unaffected by exposure to definitions, except those containing probability thresholds; (c) mostly, though not entirely, resembles an expected harm calculation; and (d) differs widely between abstract and concrete scenarios but not across concrete risk type (such as climate change, pandemics, and nuclear war). These results provide crucial insights for those tasked with drafting and interpreting existential risk laws, and for the coherence of ordinary meaning analysis more generally. This study also lays the foundation for a new research program we refer to as “ex ante ordinary meaning analysis”—focused not only on how judges can and should interpret legal provisions once they have been drafted, but on how lawmakers can and should draft legal provisions so as to best achieve their policy aims.