A master mariner is a licensed mariner who holds the highest grade of seafarer qualification; namely, an unlimited master's license. Such a license is labelled unlimited because it has no limits on the tonnage, power, or geographic location of the vessel that the holder of the license is allowed to serve upon. A master mariner would therefore be allowed to serve as the master of a merchant ship of any size, of any type, operating anywhere in the world, and it reflects the highest level of professional qualification amongst mariners and deck officers.
The term master mariner has been in use at least since the 13th century, reflecting the fact that in guild or livery company terms, such a person was a master craftsman in this specific profession (e.g., master carpenter, master blacksmith).
The Extra Master's qualification (issued only in the United Kingdom), which was discontinued in the 1990s, used to be the highest professional qualification and it was the pinnacle for any mariner to achieve. There are also various other levels of master's certificates, which may be restricted or limited to home trade/near coastal voyages and/or by gross tonnage. The holder of a restricted master's certificate is not referred to as a "master mariner".
In the British Merchant Navy a master mariner who has sailed in command of an ocean-going merchant ship will be titled captain. A professional seafarer who holds a restricted or limited master's certificate who has sailed in command of a ship (i.e. appropriate to the size, power or geographic limits of their certificate) can also be titled captain.
In the UK, an unrestricted master's certificate is colloquially called a "master's ticket" or a "master's." It is sometimes still referred to as a "Class 1" or "Master Foreign-Going" as it was named during the latter part of the 20th century. The conventions or acts governing the certificate have evolved alongside the shipping industry and the official name of a master mariner's qualification has varied over the years. The UK Maritime & Coastguard Agency, in line with the amended STCW convention, currently[when?] title the certificate Master Unlimited.
The qualification is awarded to those senior ships officers whose competency has been assessed under the STCW A-II/2 syllabus.
In the United States, the term was introduced in the mid-19th century.
A Master's license states the area of operations and maximum gross tonnage of vessel for which that license is valid. "Master, Any Gross Tons, Oceans" is the highest level license that it is possible to achieve. Holding this license, a person is then referred to as a 'Master Mariner' or 'Unlimited Master'. Every commercial vessel must have a 'Master', even a small sight-seeing tour boat operating on a Lake. The license required for that vessel would have restrictions noted on the license, for example "Master, Not more than 100 GRT upon Inland Waters". There are tens of thousands Master's licenses issued by the United States Coast Guard, but perhaps less than one thousand which are Unlimited Master's. The Master of any vessel is generally referred to as 'Captain'.