Obligatory

a.

1.

Binding in law or conscience; imposing duty or obligation; requiring performance or forbearance of some act; - often followed by on or upon; as, obedience is obligatory on a soldier. As long as the law is obligatory, so long our obedience is due. - Jer. Taylor.

Adj.	1.	obligatory - morally or legally constraining or binding; "attendance is obligatory"; "an obligatory contribution" Antonyms: optional - possible but not necessary; left to personal choice
	2.	obligatory - required by obligation or compulsion or convention; "he made all the obligatory apologies"

absolute, binding, choiceless, compulsory, conclusive, de rigueur, decisive, decretory, demanded, dictated, entailed, essential, exigent, final, hard-and-fast, imperative, imperious, importunate, imposed, incumbent, indispensable, inevitable, involuntary, irrevocable, mandated, mandatory, must, necessary, necessitous, peremptory, prescript, prescriptive, required, requisite, ultimate, urgent, without appeal, without choice