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Definition from Nolo’s Plain-English Law Dictionary

An attestation that one will tell the truth, or a promise to fulfill a pledge, often calling upon God as a witness. The best known oath is probably the witnesss pledge to tell the truth, the whole truth, and nothing but the truth during a legal proceeding. All witnesses are given the oath (sworn in) before testifying. In another context, a public official usually takes an oath of office before assuming the position, declaring that he or he will faithfully perform the job's duties.

Definition provided by Nolo’s Plain-English Law Dictionary.