Synonyms and related words :
absolute, binding, called for, compulsatory, compulsory, conclusive, de rigueur, decisive, decretory, dictated, entailed, essential, final, hard-and-fast, imperative, imperious, imposed, indicated, indispensable, inevitable, involuntary, irreducible, irreductible, irreplaceable, irrevocable, mandated, mandatory, must, necessary, needed, needful, obligatory, peremptory, prerequisite, prescript, prescriptive, requisite, ultimate, unforgoable, vital, wanted, without appeal