The nodachi, or Odachi as it was sometimes called, was a weapon of the calvary and foot solders of ancient Japan.
The japanese "tachi" refers to a kind of sword. (info on this sword is also available on this web page.) The prefix "o" is an honorific which implies that a person or object has some special significance. In this case, the significance is in the size of the sword. Often near or over five feet long with a tsuka of greater than 15 inches, the nodachi were used in several ways.
One use of the nodachi was to fight from horseback. Often it was used in place of a polearm because the length of it's blade made it perfect for cutting down an enemy and his horse in one fell swoop. Because of it's weight it could not be brandished with ease and was usually discarded when the melee began. Foot soldiers would carry this sword with the flat edge against the shoulder and the fuchi, or butt of the tsuka, in the palms of the hands and the blade facing out toward the enemy. The sword would be hurled at the enemy, sometimes cutting down several enemy soldiers at once. After having thrown his nodachi away, the samurai would draw his katana for the melee.