One Piece

One Piece (glossed in print as ひとつなぎの大秘宝 Hitotsu nagi no Daihihō "The Great Treasure of One Piece", but still pronounced Wanpīsu) is a legendary treasure said to be of unimaginable value. It is said that it was hidden somewhere deep in the Grand Line by its previous owner, the Pirate King Gol D. Roger.

The Mystery of the Great Treasure
Nobody seems to know what it really is or what it looks like. Given the recurring theme of treasure being something of great personal value, One Piece may simply be an item that Gol D. Roger valued.

The closest the Straw Hats come to finding it out so far was when Usopp asked Rayleigh what it was. However at that moment was interupted by Luffy who voiced a disapproval at finding out now what the treasure was, preferring to find out the nature of One Piece by finding the treasure himself.