One Piece

One Piece (ひとつなぎの大秘宝（ワンピース）) is a legendary treasure said to be of unimaginable value.

It is said that it was hidden somewhere deep in the Grand Line, presumably in the last island, Raftel, by its deceased owner, the Pirate King Gol D. Roger.

The Mystery of the Great Treasure
Aside from the Roger Pirates and Whitebeard, before their death, no one actually knows where the great treasure is.

Given the recurring theme of treasure being something of great personal value (for example Luffy's hat), One Piece may simply be an item that Gol D. Roger valued. In Buggy's flashback to his days as a member of the Black Hair Pirates, he discussed with Shanks the meaning of a treasure, and he found that all the crew shared the same views regarding personal value.

The closest the Straw Hat Pirates have ever come to finding out the nature of One Piece was during the Sabaody Archipelago Arc when Usopp tried to ask Silvers Rayleigh about it. However, Luffy stopped Usopp from discussing the treasure on the grounds that it would defeat the purpose of their adventures, and that becoming the Pirate King would have little merit if he already knew about the One Piece.

After constant speculations and doubts, the existence of the treasure was confirmed by Whitebeard with his last breath. He mentioned that "a grand battle that engulfs the entire world" and "the world will be shaken to its core" when One Piece is found.

Trivia

 * In the opening Jungle P, there is a clip where the Straw Hat Pirates sit near a treasure, and one of the treasure has the words "One Piece" written on it.