An ōdachi (大太刀), meaning "great/thick sword", was a type of Japanese greatsword.
The term nodachi, or "battle sword", which refers to a different type of sword, is often incorrectly used instead of Ōdachi. It is historically known as ōtachi.
The character for ō (大) means "big" or "huge". The character for da (太) and chi (刀) are the same as tachi (太刀), the old style of swords predating the katana.
chi is also the same character as in katana(刀) and the tō for nihontō (日本刀 "Japanese sword"), originally coming from the Chinese character for knife, dāo.
To be considered an ōdachi, the sword in question must have a blade length of 3 chaku (90.9 cm) or more. However, as with many terms in the Japanese sword art, there is no exact definition of the size of an ōdachi.
The purpose of the ōdachi can be categorized as follows:
The ōdachi are difficult to produce because their length makes heat treatment more complicated in the traditional way: the longer a blade is, the more difficult (or expensive) it is to heat the whole blade to a homogeneous temperature, both for annealing and to reach hardening temperature.
Then the tempering process needs a higher tempering medium as uneven tempering can lead to twisting of the blade.
The polishing method is also different. Because of their size, ōdachi are usually hung from the ceiling or placed in a stationary position to be polished, unlike normal swords that are moved on polishing stones.
Acquiring an ōdachi would be very difficult as they would most certainly have to be custom made. Thus, the local government or a religious organization would have to sponsor production, as there is no reason for their creation otherwise.
Ōdachi's Form of use
The ōdachi that were used as weapons were too long for samurai to carry on their waists like normal swords. There were two methods of carrying them:
The importance of the ōdachi faded after the Osaka siege of 1615 (the battle between Tokugawa Ieyasu and Toyotomi Hideyori). From then on it was used more as a ceremonial piece.