sine qua non
I
noun
absolute condition, absolute prerequisite, antecedent, condition, contingency, essential clause, essential condition, essential matter, essential part, essential qualification, indispensable condition, indispensable item, necessity, precondition, prerequisite, prime constituent, prime ingredient, qualification, requirement, vital concern
II
index
center (essence), cornerstone
Burton's Legal Thesaurus.
William C. Burton.
2006