An oath is a formal promise, especially a promise to be loyal to a person or country. [...] 2. See also Hippocratic oath. 3. 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. [...] More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.