indispensable Synonyms

essential vitally important significant right-hand requisite required principal primary pressing prerequisite pivotal paramount obligatory needful needed need structural substantive unavoidable of vital importance unreplaceable must-have irreplaceable unassignable obbligato life-or-death life-and-death axiological of the essence all important vital urgent underlying necessitous necessaries fundamental focal demanded crucial critical compulsory compelling clamant chief central cardinal bounden basilar basic imperative