Daisho (Katana and Wakizashi) was a set of long and short swords, worn by samurai (Japanese warriors) during the Edo period. A long sword is called a Katana, and a short sword is called a Wakizashi.
Daitō means long sword, and shōtō means short sword. daitō + shōtō = daisho. The daishō is usually depicted as a katana and wakizashi (or tanto) mounted in their respective koshirae, but originally a daishō was the carrying of any long and short blade together. The katana / wakizashi pairing is not the only daishō combination, as generally any longer sword paired with a smaller one is considered a daishō. Daishō eventually came to mean two swords with matching koshirae.