obligatory

adjective

1 use of seat belts in cars is now obligatory:

compulsory, mandatory, prescribed, required, demanded, statutory, enforced, binding, incumbent;

requisite, necessary, imperative, unavoidable, inescapable, essential.

▷antonyms voluntary, optional.

2 after the obligatory preamble on the weather he got down to business:

customary, traditional, usual, accustomed, routine, familiar, regular, habitual;

French de rigueur;

literary wonted.

obligatory Oxford Dictionary of English