See requisite in Webster's New World Roget's A-Z Thesaurus II
adjective
Incapable of being dispensed with:
essential, indispensable, necessary, needful, required. See important, necessaryImposed on one by authority, command, or convention:
compulsory, imperative, mandatory, necessary, obligatory, required. See obligation
noun
Something indispensable:
condition, essential, must, necessity, need, precondition, prerequisite, requirement, sine qua non. See necessary
Learn more about requisite