indispensable

noun 0.632

prerequisite

noun 0.610

indispensability

noun 0.602

requisite

noun 0.598

must-have

noun 0.580

nonnegotiable

noun 0.577

sufficient condition

noun 0.569

necessity

noun 0.566

essential

noun 0.566

precondition

noun 0.551

sui generis

noun 0.542

prereq

noun 0.533

nonessential

noun 0.533

condition precedent

noun 0.527

necessitation

noun 0.522

sine

noun 0.522

desideratum

noun 0.520

quintessence

noun 0.517

necessitarian

noun 0.516

requirement

noun 0.514

quiddity

noun 0.513

unconditional

noun 0.512

perquisite

noun 0.511

paramount

noun 0.509

incomparable

noun 0.509

mandatory

noun 0.508

conditional

noun 0.507

quod non

noun 0.507

obbligato

noun 0.506

invariable

noun 0.506

ne plus ultra

noun 0.505

one of a kind

noun 0.504

inevitable

noun 0.503

nonfungible

noun 0.502