indispensable Synonyms

essential crucial required key necessary fundamental invaluable imperative vital unavoidable critical irreplaceable must-have principal paramount structural staple significant pivotal prerequisite pressing right-hand prime requisite primary substantive sufficient vitally important of vital importance unreplaceable nonnegotiable unassignable obbligato life-or-death life-and-death axiological of the essence all important wanted valuable urgent underlying obligatory needful important basal focal desideration demanded defining