UK /əˈblɪɡ.ə.tər.i/ US /əˈblɪɡ.ə.tɔːr.i/

C1 If something is obligatory, you must do it because of a rule or law, etc.:
The medical examination before you start work is obligatory.
[ + to infinitive ] The statute made it obligatory for all healthy males between 14 and 60 to work.

C2 expected because it usually happens:
Several Secret Service agents surrounded the President, all wearing the obligatory raincoat and hat.