authoritatively ordered; obligatory; compulsory

It is mandatory that all students take two years of math.

pertaining to, of the nature of, or containing a command.

Law . permitting no option; not to be disregarded or modified

a mandatory clause.

having received a mandate, as a nation.

mandatary.

adjective

optional , unnecessary , voluntary

adjective

binding , commanding , compelling , compulsatory , compulsory , de rigueur , essential , forced , imperative , imperious , indispensable , involuntary , irremissible , needful , obligatory , requisite , necessary , coercive , commandatory , directory , exigent , impelling , preceptive , prerequisite , required