term:

Essence

essence n

1 : the real or ultimate nature of a thing
: the properties that make a thing what it is [his award is legitimate only so long as it draws its from the collective bargaining agreement “United Steel Workers v. Enterprise Wheel and Car Corp., 363 U.S. 593 (1960)”] see also essence test
2 : the predominant purpose of a thing [the of the contract]

of the essence
: of the utmost importance
;specif
: so material in nature that failure to satisfy its requirements constitutes a breach of contract [time is of the essence]