required as a matter of obligation; mandatory

A reply is desirable but not obligatory.

incumbent or compulsory (usually fol. by on or upon )

duties obligatory on all.

imposing moral or legal obligation; binding

an obligatory promise.

creating or recording an obligation, as a document.

adjective

nonessential , optional , unrequired , voluntary , discretional

adjective

binding , coercive , compulsatory , compulsory , de rigueur , enforced , imperative , imperious , mandatory , necessary , requisite , unavoidable , essential , incumbent , irremissible , required