ob ‧ lig ‧ a ‧ to ‧ ry /əˈblɪɡət ə ri $ -tɔːri/ BrE AmE adjective
1 . formal something that is obligatory must be done because of a law, rule etc SYN compulsory , mandatory
it is obligatory for somebody (to do something)
It is obligatory for companies to provide details of their industrial processes.
2 . [only before noun] used humorously to describe something that is always done or included in a particular situation:
She offered him the obligatory cup of tea.