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.
Update
Added on
From The Tyranny of Clichés:
…an idea that assumes with Hegelian orthodoxy that expansions of the State are the sine qua non of progress.
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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.