sine qua non

Definition from Nolo’s Plain-English Law Dictionary

Latin for "without which it could not be," an indispensable action or condition. Example: if Charlie had not left the keys in the ignition, his ten-year-old son could not have started the car and backed it over Polly's bike. So Charlie's act was the sine qua non of the damage to Polly's bike.

Definition provided by Nolo’s Plain-English Law Dictionary.