Sine qua non = essential condition or requirement. It comes from the Latin, pronounced sinni kua non "A complete knowledge of spices is the sine qua non of a good cook."

Sine qua non is a Latin phrase - the language of old Rome. It means precisely "Without which nothing" — if you don't have the thing referred to as a sine qua non then you will have nothing. It is not in common use, but is an elegant expression.