What’s the difference between a ship’s captain and a ship’s master? In contemporary usage, not much, but historically, the titles represented quite distinct roles. Captain is more common in modern usage, but master is more historically accurate.
Captain derives ultimately from the Latin term caput, meaning “head” and related to other words beginning with cap- such as capital; capitaneus, meaning “chief,” and the French word capitaine, meaning “leader,” are the intermediate forms. Master, originating in the Latin word magis, meaning “more,” stems from magister, which also means “chief” as well as “director” or teacher.”
In the Middle Ages, the man in charge of a ship, often but not always the owner, was called the master; this usage stems from imperial Rome. On the high seas, where delay or dissension could lead to destruction and death, the master had absolute authority, hence the title.
However, before the advent of standing navies, civilian ships were often offered or impressed for use in transporting soldiers and their supplies from one place to another. When the captain of a company of soldiers brought the troops aboard for transport, he assumed military command of the ship, determining its destination and, if the ship engaged in hostilities at sea, directing the battle. In matters of sailing and maneuvering, though, the captain deferred to the master, who of course remained on board. (Before captain became a specific military rank, it designated a leader of roughly up to several hundred soldiers assembled for a specific battle or campaign. Much later, it became an official designation for a standing military unit of a circumscribed similar size.)
As civilian ships were given long-term military roles, this division of responsibilities remained in effect: A captain had overall authority, but the master was responsibility for sailing operations. When permanent navies were established, roughly coincident with the establishment of standing armies, captain became a precise military rank in both settings. Master was also codified as the label for a specific posting; the master (sometimes called the sailing master) oversaw navigation and provisioning of the ship under direction of the captain, who by this time was a skilled mariner as well as a military leader.
Naval captains and those commanding land-based units were and are not equivalent in rank: The largest ships in the Age of Sail eventually carried a thousand or more crew members, and ships often operated independently, requiring the captain to exercise great responsibility (and life-or-death authority), whereas an army captain commanded no more than a couple hundred men and was part of a more restrictive chain of command. Therefore, to this day, a navy captain holds a fairly high status in the naval hierarchy, whereas an army captain (or an officer with the equivalent rank in an air force or in the marines) has a comparatively minor role, though he or she must still demonstrate leadership skills.
Even in the Middle Ages, a captain designated one or more lieutenants to assist him with command, to lead the company if he was absent or incapacitated, or to temporarily take responsibility for part of the unit. (The word lieutenant comes from the French phrase lieu tenant, meaning “deputy,” from words meaning “place” and “holder.”) Later, a ship was assigned one or more lieutenants depending on the size of the vessel. Meanwhile, master also briefly became a naval military rank below the rank of lieutenant, rather than a position, but it was phased out as naval vessels were increasingly powered by steam rather than sail.
Other words that apply to naval command include commander, from an Old French word meaning “one who commands.” (The rank was originally styled “master and commander” to denote the commanding officer of a small vessel who doubled as the master, hence the Patrick O’Brian novel of that title and the related film starring Russell Crowe.) Commodore, the Dutch word derived from this term, came to apply to a captain given temporary command of a group of vessels and later became a specific rank above that of captain. (The similar term commandant is not a rank; it applies to an officer of any rank who commands a training facility or a prison.)
Admiral, meanwhile, referring to a high-ranking naval officer in command of an entire navy or a fleet or major unit within one or the other, is from the Arabic word amir, meaning “military commander” (the source of emir, a modern word for an Arab leader) and, like captain, originally pertained to a land-based leader before it was applied to one who leads naval operations.
On many modern civilian ships, the person in charge is officially referred to as the captain, whether or not there is a command hierarchy more or less based on naval tradition, though master is also common. In popular usage, however, captain came to prevail over master, so that, even now, the owner of a small pleasure craft will be referred to as “Captain” or “Skipper”; the latter word is derived from the Dutch word scipper, meaning “operator of a ship.” Skipper is also used formally to refer to the master of a small vessel such as a tugboat.
Captain is used in other civilian command hierarchies, as in police and fire departments and the like, as well as to refer to the acknowledged leading athlete on a sports team, but master has not been adopted in such contexts from nautical usage.