Sine qua non
is a Latin phrase that translates literally into "without which not." When used in English prose, it refers to an essential condition or qualification; an indispensible thing or absolute prerequisite.
In Theodore Dreiser's Sister Carrie, middle-aged George Hurstwood contemplates the state of his post-divorce life with young and pretty Carrie:
[Hurstwood] soon found that freedom from fear of arrest was not the sine qua non of his existence. That danger dissolved, the next necessity became the grievous thing. The paltry sum of thirteen hundred and some odd dollars set against the need of rent, clothing, food, and pleasure for years to come was a spectacle little calculated to induce peace of mind in one who had been accustomed to spend five times that sum in the course of a year.