obligation [ˌɒblɪˈgeɪʃn]

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n.（法律、道德上的）义务，责任

when you are obliged to do something

Every player is under an obligation to keep the rules. 每个选手都必须遵守这些规则。
It is the obligation of citizens to perform military service. 服兵役是每个公民的义务。