The character for ō (大) means "big" or "great". The characters for da (太) and chi (刀) are 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.