'Opponent' in the Bible

Each one of them seized his opponent by the head and thrust his sword into his opponents side; so they fell down together. Therefore that place was called Helkath-hazzurim [that is, the Field of Sides], which is in Gibeon.

For though I were righteous, I could not answer.I must appeal for mercy to my Opponent and Judge.

sound and beyond reproach in instruction, so that the opponent [of the faith] will be shamed, having nothing bad to say about us.