Obligatory meaning

ə-blĭgə-tôrē, ŏblĭ-gə-
adjective
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Required as part of a routine course of action or cultural expectation; predictable.
adjective
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Of the nature of an obligation; compulsory.

Attendance is obligatory. Mathematics is an obligatory course.

adjective
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Legally, morally, or socially binding; constituting, or having the nature of, an obligation; required.
adjective
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(biology) Obligate.
adjective
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Imposing obligation, morally or legally; binding: an obligatory promise.
adjective
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Requiring a matter or obligation.
adjective
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The definition of obligatory is necessary, required or expected as a result of law or social custom.

When someone waves at you and you feel like you have to wave back because social system says you do in order to be polite, the return wave is an example of an obligatory wave.

When the law requires you to stop at a red light, this is an example of a time when stopping is obligatory.

adjective
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(biology) Obligate.
adjective
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Origin of obligatory