What is "odachi"

n. A long Japanese sword built like the katana but made to be longer (about 4 to 6 feet long) for combat on horseback

An (large/great sword) or nodachi (野太刀, field sword) was a type of traditionally made Japanese sword (nihonto) used by the samurai class of feudal Japan. The Chinese equivalent and 'cousin' for this type of sword in terms of weight and length is the Miao dao, and the Western battlefield equivalent (though less similar) is the Spanish Espada Bastarda or Scottish Claymore.

The character for ō (大) means "big" or "great". The dachi here (太刀) is the same as tachi (太刀, lit. "great sword"), the older style of sword/mounts that predate the katana. The chi is also the same character as katana (刀) and the tō in nihontō (日本刀 "Japanese sword"), originally from the Chinese character for a blade, dāo.

To qualify as an ōdachi, the sword in question would have a blade length of around 3 shaku (35.79 inches or 90.91 cm); however, as with most terms in Japanese sword arts, there is no exact definition of the size of an ōdachi.