Katana

Katana (刀) is the word for "sword" in the Japanese language. It is also used specifically for a type of Japanese backsword or longsword (大刀 daitō) in use after the 1400s: a curved, single-edged sword traditionally used by the samurai. Pronounced [kah-tah-nah] in the kun'yomi (Japanese reading) of the kanji 刀, the word has been adopted as a loan word by the English language; as Japanese does not have separate plural and singular forms, both "katanas" and "katana" are considered acceptable plural forms in English.

One Piece
In context of real life pirates, the presence of katana in the show is somewhat misplaced. However, being a Japanese anime (and taking into account the many pirates that were sailing when people from Europe travelled to Japan) they are acceptable in terms of the storyline reguardless.

The Katana in One Piece come in several different grades - Saijō Ō Wazamono Grade Swords, Ō Wazamono Grade Swords, Ryō Wazamono Grade Swords, Wazamono Grade Swords. Roronoa Zoro typically wields 3 katana at once. Other characters who have been seen using Katanas include Kuina (Zoro's childhood friend) and the marine officer Tashigi. Outside of a few filler characters, Katana have so far rarely been seen in the story with more generic nameless swords making appearances in general.