Sine qua non
Latin for “without which, not.” It’s used to refer to something indispensable – “without this thing, this other thing is not possible.”
Why I Looked It Up
I can’t remember the specific usage that caused me to look it up, but I’ve seen this for years.
Postscript
Added on July 1, 2023
From The Tyranny of Clichés:
…an idea that assumes with Hegelian orthodoxy that expansions of the State are the sine qua non of progress.