obligatory

1. Morally or legally constraining or binding (synset 300851498)
   "attendance is obligatory"; "an obligatory contribution"
   also: necessary - absolutely essential
   similar to:

   • bounden - morally obligatory
   • compulsory, mandatory, required - required by rule
   • de rigueur - required by etiquette or usage or fashion
   • imposed - set forth authoritatively as obligatory
   • incumbent on - morally binding or necessary
   • indispensable - unavoidable
   • prerequisite - required as a prior condition or course of study
2. Required by obligation or compulsion or convention (synset 301585376)
   "he made all the obligatory apologies"
   similar to: necessary - absolutely essential

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