A type of sword used by samurai before the advent of the katana, primarily used on horseback.
Tachi and katana generally differ in:
Curvature: tachi tend to be more curved (both hilt and blade).
How they are worn: tachi is worn suspended from the belt with its cutting edge downward, while the katana is thrust through the belt with its cutting edge upward.
Length: tachi tend to be longer than the katana.
Hilt: while katana always have a rayskin-wrapped hilt with silk overwrapping, tachi sometimes only have the rayskin wrapping, though this is far from universal.
Examples
Above: Tachi Below: Katana
See also
Chokutou, straight swords, were also called tachi, but written as 大刀