/ əˈblɪgətri; NAmE -tɔːri/ adjective
1.
obligatory (for sb) (to do sth) ( formal ) that you must do because of the law, rules, etc.
SYN compulsory :
It is obligatory for all employees to wear protective clothing.
OPP optional
2.
(often humorous ) that you do because you always do it, or other people in the same situation always do it :
In the mid 60s he took the almost obligatory trip to India.
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WORD ORIGIN
late Middle English : from late Latin obligatorius , from Latin obligat- obliged, from the verb obligare , from ob- towards + ligare to bind.