properly the sovereign or governor of the fourth part of a country. (Matthew 14:1; Luke 3:1; 9:7; Acts 13:1) The title was, however, often applied to any one who governed a Roman province, of whatever size. The title of king was sometimes assigned to a tetrarch. (Matthew 14:9; Mark 6:14,22)
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Smith, William, Dr. "Entry for 'Tetrarch,'". "Smith's Bible Dictionary".