Definition
In law, "captain" carries two distinct meanings depending on context:
1. MARITIME: The commander of a vessel — whether a warship or merchant ship — responsible for its navigation, crew, and cargo. In admiralty and maritime law, the preferred legal term for this role is master, not captain. The word captain is used colloquially by crew and officers aboard merchant vessels but carries less precise legal weight than master in admiralty proceedings and contractual instruments.
2. MILITARY: A commissioned officer rank. In the United States Army, Marine Corps, and Air Force, captain designates a company-grade officer ranking above first lieutenant and below major. In the United States Navy and Coast Guard, captain is a senior field-grade equivalent, ranking above commander and below rear admiral (lower half), and is roughly equivalent to a colonel in the land services.
Common Language
Modern common usage (Wiktionary): An honorific applied to a ship's commanding officer or a person holding the military rank of captain; also used loosely as a title or nonce nickname.
Historical common usage (Webster's 1913): A head or chief officer; specifically, the military officer commanding a company, troop, or battery, or by courtesy the officer actually commanding a vessel regardless of formal rank.
Editorial note: Common usage treats "captain" as a general honorific for anyone commanding a vessel or holding a leadership role. Legal usage — particularly in admiralty law — draws a firm distinction: the commanding officer of a merchant vessel is legally the master, with specific duties and liabilities defined by that title. Calling someone a "captain" in everyday speech does not automatically invoke the legal status, duties, or liabilities of a master under maritime law.
Common Confusion
Captain vs. Master: In admiralty and maritime law, these terms are not interchangeable despite referring to the same person aboard most merchant vessels. Master is the operative legal title that governs the officer's agency relationship with the shipowner, authority over crew and cargo, and liability exposure. Captain is the customary and colloquial title. Legal documents, charter parties, bills of lading, and admiralty pleadings use master. A researcher encountering "captain" in a maritime legal instrument should read it as a functional equivalent to master unless the context specifies otherwise, and should then look to the doctrine governing masters for the applicable rules.
Captain (Navy) vs. Captain (Army/Marines/Air Force): These are structurally different ranks. A naval captain commands a major warship and is equivalent in pay grade to a colonel. An army captain commands a company and is equivalent to a lieutenant in some other nations' structures. Research touching military law, courts-martial, or chain-of-command questions must be precise about service branch.
Why It Matters in Research
The captain/master distinction is the primary research trap in this entry. Admiralty treatises and historical maritime cases consistently use master as the operative legal term, and the doctrines attached to that title — including the master's authority to bind the shipowner by necessity, the master's duty of care to seamen, and the master's role in protests and average adjustments — are indexed under master, not captain. A researcher searching historical maritime sources for "captain" may miss the controlling authority if they do not also search under master.
For military law research, the rank of captain appears in courts-martial records, pension files, and military commission proceedings. In historical sources, the rank structure and corresponding authority varied by branch and era. Nineteenth-century sources may use captain to describe naval officers of varying formal ranks who happened to be commanding a vessel, compounding the ambiguity.
In agency and tort contexts, the question of whether a "captain" acts as agent or employee of a shipowner — and thus whether the owner bears respondeat superior liability — turns on the master doctrine, not on whatever title the parties used colloquially. Courts look past the label.
Historical Dictionary Support
Black's Law Dictionary (1st and 2nd editions) and Burrill's Law Dictionary agree on the core point: captain in the maritime context is the commanding officer of a vessel, but master is the legally operative term in admiralty and maritime business. Burrill makes this explicit, noting that the commander of a ship is "more commonly termed in law, master." Black's frames it descriptively, observing that "captain" is the term used by inferior officers and seamen but that maritime business and admiralty law prefer master.
Both Black's editions describe the naval rank structure (commander — captain — commodore), which reflects the nineteenth-century U.S. Navy hierarchy before the later introduction of the rank of rear admiral and the reorganization of flag officer grades. Researchers using these sources for naval rank questions should verify the contemporary rank structure, as it has changed.
None of the historical sources address the land-service military meaning of captain in depth, nor do they explore the agency and liability dimensions that make the captain/master distinction consequential in litigation. For those questions, researchers should move from dictionary sources to treatises on admiralty law.
Jurisdictional Note
In U.S. federal admiralty law, master is the controlling term and carries defined legal duties under federal statute and maritime common law. State courts applying general maritime law follow the same convention. In British admiralty sources — which heavily influence American maritime law — master is equally dominant, though older English materials may use captain more freely in narrative contexts.