A captain is the person in command of a ship, aircraft, or military unit. The captain makes the decisions, bears the responsibility, and answers for what happens.
On a ship, the captain is called the master. The master’s authority comes from being formally assigned to command — not from rank, seniority, or ownership. A ship owner who has not been installed as master has no command authority aboard. A junior officer who has been installed as master has full command authority. Command is a position, not a personal attribute.
The captain is personally accountable for everything that happens within scope. US Navy Regulations, Art. 0802: “delegation of authority shall in no way relieve the commanding officer of continued responsibility.” The captain can delegate tasks but not accountability. If a subordinate makes a mistake, the captain is responsible for having delegated to that person.
Certain duties only the captain can perform. Signing the official log. Making navigational decisions in an emergency. Conducting disciplinary hearings. Responding to distress signals. These are non-delegable — having someone else do them does not count as having done them.
The captain’s authority has limits but within those limits it is absolute. SOLAS V/34-1: “the owner, the charterer, the company… shall not prevent or restrict the master” from making safety decisions. A superior who wants to override the captain must formally take command — assume the position and the accountability that comes with it.
Command transfers through a formal act. In the navy: “I relieve you” / “I stand relieved,” the passing of colors, a published order. In merchant marine: entry in the official log. The moment is discrete — before it, the old captain commands; after it, the new one does. No overlap, no gap. If the captain dies or is incapacitated, the next in line assumes command immediately and documents it afterward. Command never lapses.
The word comes from Latin caput, head. The naval sense (commander of a ship) and the military rank (O-3 in the US Army, O-6 in the US Navy) are different things sharing a word. An Army captain may or may not command a company. A Navy captain may or may not command a ship. The rank and the position are separate.