The Oxford Thesaurus An A-Z Dictionary of Synonyms - indispensableadj.

1) crucial, vital, essential, urgent, imperative, necessary, needed, needful, required, requisite, demanded, in demand, called-for, of the essence, important, compelling: A good television manner is indispensable in a national leader these days. 2) key, unavoidable, inescapable, ineluctable, compulsory, mandatory, sine qua non, obligatory: Are a black bowler and tightly furled umbrella still indispensable to doing business in the City?