Sine qua non, meaning an indispensable element, is a loanword from Latin, translating roughly to without which not. It’s always a noun, usually italicized (although italicization is not necessary), and it’s usually preceded by the or a and followed by of.
As with all rare loanwords, sine qua non works when you’re reasonably certain most of your audience will know what it means. Elsewhere, using an English alternative is usually best (in this case, e.g., prerequisite, requirement, essential condition).
While an allegation that a tenant primarily resides at a specified alternate location is not the sine qua non of pleading sufficiency in every nonprimary residence case (see Price v Chelsmore Apts., NYLJ, March 8, 1996, at 25, col 2 [App Term, 1st Dept]), the absence of such an allegation was fatal in these circumstances. [quoted on Justia]
Evidence provided by the fossil record, primate behavior, and demographic analysis shows that the traditional view that early human evolution was a direct consequence of brain expansion and material culture is incorrect, and that the unique sexual and reproductive behavior of man may be the sine qua non of human origin. [Science Magazine]