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

From The Tyranny of Clichés:

…an idea that assumes with Hegelian orthodoxy that expansions of the State are the sine qua non of progress.

Postscript

From Harry Potter and the Art of Spying:

What is sine qua non? A wonderful Latin phrase that means an essential action, condition, or ingredient; as a legal term, it means a condition or preexisting ingredient without which that which follows cannot exist.