A navy, naval force, or maritime force 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. The strategic offensive role of a navy is projection of force into areas beyond a country's shores. 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, open-ocean applications, and something in between, although these distinctions are more about strategic scope than tactical or operational division.

नौसेना (Navy) किसी देश की संगठित समुद्री सेना को कहते हैं। इसके अंतर्गत रणपोत, क्रूज़र (cruiser), वायुयानवाहक, ध्वंसक, सुरंगें बिछाने तथा उन्हें नष्ट करने आदि के साधन एवं सैनिकों के अतिरिक्त, समुद्रतट पर निर्माण, देखभाल, पूर्ति तथा प्रशासन, कमान, आयोजन और अनुसंधान संस्थान भी सम्मिलित हैं। इस प्रकार नौसेना सरकार के सीधे नियंत्रण में स्थित सैनिक संगठन है, जिसके प्रशासन के लिए सरकार पृथक् विभागों की स्थापना करती है।