a worker for a boss- usually w/o pay
This is one among a number of titles of the Pope.
domestic worker (a person working in the service of another (especially in the household))
The pope is called this because he spends his whole life on the catholic church so the apostles are the servants of god and the pope is the apostle's servant because he spreads on the catholic faith...
Once he becomes king, Macbeth has many servants. His personal servant is Seyton (pronounced just like "Satan").