An essential element or condition; pre condition; "without which it could not be"; This phrase has strayed into general language from Law.

A leaves his car with the key in the ignition. A child starts the car and hits C. Without A’s careless act, the child could not have driven the car. A’s act was condition Sine qua non of C’s injury.

From judgements:

“The provisions of the Act mandate that a finding that the goods are marketable is a prerequisite or “sine qua non” for the levy of duty”.

The sine qua non of a good and efficient administration is that it is fair and transparent.

But it is a sine qua non that the liability to pay service tax should be, in the first instance, fixed on the non-resident service-provider so that the tax could be demanded from the service-recipient in India.