Commander (often abbreviated "Cdr") is a senior officer rank of the Royal Navy of the United Kingdom. It is immediately junior to captain and immediately senior to the rank of lieutenant commander. Officers holding the junior rank of lieutenant commander are not considered to be commanders.

The title (originally "master and commander") originated in the 18th century to describe naval officers who commanded ships of war too large to be commanded by a lieutenant but too small to warrant the assignment of a post-captain, or (before about 1770) a sailing-master; the commanding officer served as his own master. In practice, these were usually unrated sloops-of-war of no more than 20 guns. The Royal Navy shortened "master and commander" to "commander" in 1794; however, the term "master and commander" remained (unofficially) in common parlance for several years. In the 20th and 21st centuries, the rank has been assigned the NATO rank code of OF-4.

A commander in the Royal Navy is senior to an officer holding the rank of lieutenant commander but junior to a captain. A commander may command a frigate, destroyer, submarine, aviation squadron or shore installation, or may serve on a staff. Formerly equivalent to the Army rank of major, a commander is now equivalent in rank to a lieutenant colonel in the British Army or a wing commander in the Royal Air Force. The rank of wing commander was derived from the naval rank of commander via the usage in the World War I Royal Naval Air Service.

Commander is a naval rank which is also sometimes used as a military title depending on the individual customs of a given military service. Commander is also used as a rank or title in some organizations outside of the armed forces, particularly in police and law enforcement.

Commander is a rank used in navies but is very rarely used as a rank in armies (except in special forces where it designates the team leader). The title (originally "master and commander") originated in the 18th century to describe naval officers who commanded ships of war too large to be commanded by a Lieutenant but too small to warrant the assignment of a post-captain, or (before about 1770) a sailing-master; the commanding officer served as his own Master. In practice, these were usually unrated sloops-of-war of no more than 20 guns. The Royal Navy shortened "master and commander" to "commander" in 1794; however, the term "master and commander" remained (unofficially) in common parlance for several years. The equivalent American rank master commandant remained in use until changed to commander in 1838. A corresponding rank in some navies is frigate captain. In the 20th and 21st centuries, the rank has been assigned the NATO rank code of OF-4.

The Royal Navy (RN) is the naval warfare service branch of the British Armed Forces. Tracing its origins to the 16th century, it is the oldest service branch and is known as the Senior Service. From the end of the 17th century until well into the 20th century it was the most powerful navy in the world, playing a key part in establishing the British Empire as the dominant world power.

Following victory in the First World War the Royal Navy was significantly reduced in size, although at the onset of the Second World War it was still the largest in the world. By the end of the Second World War the U.S. Navy had emerged as the world's largest. During the course of the Cold War and the emergence of the Soviet submarine threat, the Royal Navy transformed into a primarily anti-submarine force, hunting for Soviet submarines, mostly active in the GIUK gap. Following the collapse of the Soviet Union its focus has returned to global expeditionary operations.

The Royal Navy operates a fleet of technologically sophisticated ships including an aircraft carrier (though without any fixed-wing aircraft), a helicopter carrier, two landing platform docks, four ballistic missile submarines (which maintain the UK's nuclear deterrent), seven nuclear fleet submarines, six guided missile destroyers, 13 frigates, 15 mine-countermeasure vessels and 24 patrol vessels. As of April 2012, there were 79 commissioned ships in the Royal Navy, plus 19 commissioned ships of the Royal Fleet Auxiliary (RFA) which also contribute to the Royal Navy's available sea-going assets. The RFA primarily serves to replenish Royal Navy warships at sea, and also augments the Royal Navy's amphibious warfare capabilities through its three Bay-class landing ship dock vessels.

Royal may refer to the following:

A navy (sometimes called a maritime force) is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions. It includes anything conducted by surface ships, amphibious ships, submarines, and seaborne aviation, as well as ancillary support, communications, training, and other fields; recent developments have included space-related operations. The strategic offensive role of a navy is projection of force into areas beyond a country's shores (for example, to protect sea-lanes, ferry troops, or attack other navies, ports, or shore installations). The strategic defensive purpose of a navy is to frustrate seaborne projection-of-force by enemies. The strategic task of the navy also may incorporate nuclear deterrence by use of nuclear missiles.

First attested in English c.1600, the word "navy" came via Old French navie, "fleet of ships", from the Latin navigium, "a vessel, a ship, bark, boat", from navis, "ship" and from Sanskrit "नाव" (Nau), "ship".^{[dead link][citation needed]} The word "naval" came from Latin navalis, "pertaining to ship";^{[dead link]} cf. Greek "ναῦς" (naus), "ship", "ναύτης" (nautes), "seaman, sailor" (the earliest attested reference to the word is the Mycenaean Greek na-u-do-mo, "shipbuilders", written in Linear B syllabic script).