I don’t know if this phrase is useful for most people to use, but it is certainly useful to know. Let us add it to our vocabulary.
Sine qua non translates to “without which (there is) nothing. It can be translated to “but for”, but it makes more sense in that longer English translation. Another useful definition is: “an indispensable element”. It is used as a noun, typically used as a determinate phrase.
The sine qua non of a business is customers.