An ōdachi (大太刀), which translates to "great/giant sword," was a type of long Japanese sword. The term nodachi, or "field sword," which refers to a different type of sword, is sometimes confused and used interchangeably with ōdachi.
The symbol for ō (大) means "big" or "large." The characters for da (太) and chi (刀) are the same as tachi (太刀, literally "big sword"). Chi is also the same character used in katana (刀) and tō in nihontō (日本刀, "Japanese sword"), originally derived from the Chinese written language for knife, dāo.
To be classified as an ōdachi, the sword must have a blade length of over 3 shaku (90.9 cm). However, like many terms in the art of Japanese swordsmanship, there is no exact definition of the dimensions of an ōdachi.