1. Morally or legally constraining or binding
   "attendance is obligatory"; "an obligatory contribution"
   - irremissible 
2. Required by obligation or compulsion or convention
   "he made all the obligatory apologies"

See also: bounden [archaic], compulsory, de rigueur, imposed, incumbent on, indispensable, mandatory, necessary, obligatorily, prerequisite, required

Antonym: optional

Encyclopedia: Obligatory

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