The title "One Piece" is derived from the main quest of the series, which is to find the legendary treasure called "One Piece." The "Z" in "One Piece Z" refers to the special edition of the manga that includes additional content, such as character interviews and commentary by the author, Eiichiro Oda. This special edition is called "Z" because it is an extension or supplement to the main series, much like a "zero" edition or a "special" edition.

The reason for using "Z" in this context is likely due to the Japanese language's use of the Roman numeral "Z" to denote a special or supplementary edition. In Japanese, "Z" is pronounced as "ゼロ" (zero), which can also be interpreted as "special" or "extra." This naming convention is consistent with other manga and anime series that have special editions, such as "Naruto Shippuden," which has a "Naruto Z" edition. The use of "Z" in "One Piece Z" signifies that this is a special collection of content that is not part of the regular manga chapters.

Data from the sales of "One Piece Z" editions and the popularity of the special content among fans support the idea that the "Z" in "One Piece Z" is a successful branding strategy. According to a report by Oricon, the "One Piece Z" volumes have consistently ranked high on the manga sales charts, indicating a strong demand for this type of supplementary material. Additionally, the inclusion of author commentary and character interviews has been well-received by fans, as it provides deeper insights into the series and its characters. This suggests that the "Z" in "One Piece Z" effectively communicates the unique value of this special edition to the target audience.