A navy or maritime force is a fleet of waterborne military vessels (watercraft) and its associated naval aviation, both sea-based and land-based. It is the branch of a nation’s armed forces principally designated for naval and amphibious warfare; namely, lake-borne, riverine, littoral, 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 submarine-launched ballistic missiles. Naval operations can be broadly divided between riverine and littoral applications (brown-water navy), open-ocean applications (blue-water navy), and something in between (green-water navy), although these distinctions are more about strategic scope than tactical or operational division.

In most nations, the term “naval”, as opposed to “navy”, is interpreted as encompassing all maritime military forces, e.g., navy, naval infantry/marine corps, and coast guard forces.