A wakizashi (脇差) is a traditional Japanese sword with a blade between 12 and 24 inches, similar to but shorter than a katana, together with which it was often worn. When worn together the pair of swords were called daisho which translates literally as large and small; dai or large for katana, and sho for wakizashi.
According to traditional samurai law, or Bushido, a wakizashi is used only to commit seppuku (i.e. hara-kiri). After the 16th century it was sometimes used in the off hand with the katana, a technique pioneered by Miyamoto Musashi.
See also tachi, tsurugi, saya.