Lookups for sine qua non spiked on May 27, 2017, following news of the death of Zbigniew Brzezinski (\zuh-BIG-nyef bruh-ZHIN-skee\), a foreign policy expert who was President Carter’s national security advisor. Some reports of his death included the text of the statesman’s last tweet, from May 4:
Sophisticated US leadership is the sine qua non of a stable world order. However, we lack the former while the latter is getting worse.
Sine qua non is the Latin phrase that literally translates to “without which not,” and is used in English as a noun to mean “the one thing that is absolutely essential,” as Brzezinski used it, and can also mean “something that is considered essential,” as in “the book is a sine qua non for word lovers.” This Latin phrase has been used in English contexts since about 1600, when Latin was still the language of academic, religious, and legal discourse and was widely understood by educated speakers of English.
Sine qua non is also used adjectivally in English to mean “absolutely necessary” (as in “sine qua nonconditions”). Conditio sine qua non is occasionally used in English to mean “an indispensable condition.” Our 1934 Unabridged dictionary also included the adjective sine-qua-nonical and the noun sine-qua-noniness.