1. （尤指对个人或国家的）誓言，誓约
    An oath is a formal promise, especially a promise to be loyal to a person or country.

    e.g. He took an oath of loyalty to the government...
           他宣誓效忠政府。
    e.g. He swore an oath promising to uphold and protect the country's laws and constitution.
           他宣誓捍卫国家的法律与宪法。

2. （在法庭上承诺说实话的）誓言
    In a court of law, when someone takes the oath, they make a formal promise to tell the truth. You can say that someone is on oath or under oath when they have made this promise.

    e.g. His girlfriend had gone into the witness box and taken the oath...
           他的女朋友进入证人席并立了誓。
    e.g. Under oath, Aston finally admitted that he had lied...
           在宣誓的约束下，阿斯顿终于承认自己撒了谎。

3. （表示愤怒、震惊的）咒骂，诅咒
    An oath is an offensive or emphatic word or expression which you use when you are angry or shocked.

oath

    e.g. Wellor let out a foul oath and hurled himself upon him.
           韦勒骂了一句粗话，然后向他猛扑过去。