Easton's Bible Dictionary
A solemn appeal to God, permitted on fitting occasions (Deuteronomy 6:13; Jeremiah 4:2), in various forms (Genesis 16:5; 2 Samuel 12:5; Ruth 1:17; Hosea 4:15; Romans 1:9), and taken in different ways (Genesis 14:22; 24:2; 2 Chronicles 6:22). God is represented as taking an oath (Hebrews 6:16-18), so also Christ (Matthew 26:64), and Paul (Romans 9:1; Galatians 1:20; Philippians 1:8). The precept, "Swear not at all," refers probably to ordinary conversation between man and man (Matthew 5:34, 37). But if the words are taken as referring to oaths, then their intention may have been to show "that the proper state of Christians is to require no oaths; that when evil is expelled from among them every yea and nay will be as decisive as an oath, every promise as binding as a vow."
Noah Webster's New International Dictionary of the English Language
1. (n.) A solemn affirmation or declaration, made with a reverent appeal to God for the truth of what is affirmed.
2. (n.) A solemn affirmation, connected with a sacred object, or one regarded as sacred, as the temple, the altar, the blood of Abel, the Bible, the Koran, etc.
3. (n.) An appeal (in verification of a statement made) to a superior sanction, in such a form as exposes the party making the appeal to an indictment for perjury if the statement be false.
4. (n.) A careless and blasphemous use of the name of the divine Being, or anything divine or sacred, by way of appeal or as a profane exclamation or ejaculation; an expression of profane swearing.