Synonyms and related words :
absolute, authoritative, binding, canonical, choiceless, commanded, commanding, compelling, compulsatory, compulsory, conclusive, de rigueur, decisive, decretory, demanded, dictated, didactic, entailed, essential, exigent, final, forced, formulary, hard and fast, hard-and-fast, imperative, imperious, importunate, imposed, indispensable, inevitable, instructive, involuntary, irrevocable, mandated, must, necessary, necessitous, needed, needful, obligatory, official, peremptory, preceptive, prescribed, prescript, prescriptive, regulation, required, requisite, rubric, standard, statutory, ultimate, urgent, without appeal, without choice