Synonyms for obligatory

Adjective

1. obligatory (vs. optional), bounden, compulsory, mandatory, required, de rigueur, imposed, incumbent on(predicate), indispensable, prerequisite, necessary

usage: morally or legally constraining or binding; "attendance is obligatory"; "an obligatory contribution"

2. obligatory, necessary (vs. unnecessary)

usage: required by obligation or compulsion or convention; "he made all the obligatory apologies"