obligatory

adjectives

morally or legally constraining or binding

"attendance is obligatory"; "an obligatory contribution"

obligatory

antonym

is similar to

morally obligatory

"my bounden duty"

is similar to

required by rule

"in most schools physical education is compulsory"; "attendance is mandatory"; "required reading"

is similar to

required by etiquette or usage or fashion

"instruction as to when and where a silk hat is de rigueur"

is similar to

set forth authoritatively as obligatory

"the imposed taxation"; "rules imposed by society"

is similar to

morally binding or necessary

"it is incumbent on me to attend"

is similar to

unavoidable

"the routine but indispensable ceremonies of state"

is similar to

required as a prior condition or course of study