kaisen

Kaisen is the romanized form of the Japanese word 海戦, which means a naval battle or sea battle. It is used in historical and military contexts to describe engagements between fleets on the ocean. The term is built from kai (sea) and sen (battle). In Japanese historical writing, battles at sea are commonly referred to as kaisen, often with a geographic qualifier, such as "対馬海戦" (Tsushima kaisen) for the Battle of Tsushima in 1905.

Outside specific occurrences, kaisen also appears in the romanization of names or titles in Japanese contexts. It may be found as part of proper nouns, including surnames or place names, when rendered in romaji. Because of similarity in spelling, it is sometimes confused with kaizen, the word for continuous improvement (改善). They are distinct in meaning and kanji.

In English-language encyclopedias, "kaisen" is typically translated as "naval battle" and used when discussing naval operations or specific battles in Japanese history.