Sine qua non
Defined in 2 dictionaries — Ballentine's (1916), Black's (1910)
Ballentine's Law Dictionary
James A. Ballentine · 1916
Without which not; an indispensable requisite.
Black's Law Dictionary
Henry Campbell Black, M.A. · 1910
Without which not. That without which the thing cannot be. An indispensable requisite or condition.
Defined under Sine in Black's Law Dictionary.