What does 'R.M.S' stand for on the Titanic?

It is the ship prefix used for seagoing vessels that carry mail under contract to the British Royal Mail. Any vessel designated as "RMS" has the right both to fly the pennant of the Royal Mail when sailing and to include the Royal Mail "crown" logo with any identifying device and/or design for the ship. It was used by many shipping lines, but is often associated in particular with the White Star Line, Cunard Line, Royal Mail Lines and Union-Castle Line, which held a number of high-profile mail contracts, and traditionally prefixed the names of many of their ships with the initials "RMS".