A person that carries a message.
God uses this word 113 times in Scripture: 109 times to Israel in the Old Testament, 3 times to the Body of Christ, and once to Israel in their tribulation, which is a retelling of an event in the Old Testament.
The messenger has no authority to speak for, or act on behalf of, the one that sends the message. They are simply the means of transport of a message.
The message can be verbal, written, or conveyed via a tangible object. In written and sealed messages, the messenger has no idea what the message is. Even when the message is verbal, or conveyed via a tangible object, the messenger can only speculate as to the actual meaning of the message.
See also Ambassador, Apostle, Christian, Disciple, Revelation, Student