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.

A licensed mariner is a sailor who holds a license from a maritime authority to hold senior officer-level positions aboard ships, boats, and similar vessels. Qualification standards for licensed mariners are universally set by the STCW Convention adopted and promulgated by the International Maritime Organization (IMO), while the licenses of individual sailors are issued by the delegated maritime authorities of the member states of the IMO; these may vary in the details of the implementation, including the government agency responsible for licensing and the local names of the grades and qualifications in each particular country.