A duty (from "due" meaning "that which is owing"; Old French: deu, did, past participle of devoir; Latin: debere, debitum, whence "debt") is a commitment or expectation to perform some action in general or if certain circumstances arise. A duty may arise from a system of ethics or morality, especially in an honor culture. Many duties are created by law, sometimes including a codified punishment or liability for non-performance. Performing one's duty may require some sacrifice of self-interest.

Cicero, an early Roman philosopher who discusses duty in his work “On Duty", suggests that duties can come from four different sources:

1. as a result of being a human
2. as a result of one's particular place in life (one's family, one's country, one's job)
3. as a result of one's character
4. as a result of one's own moral expectations for oneself

The specific duties imposed by law or culture vary considerably, depending on jurisdiction, religion, and social normalities.