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.

Another theory, is that the 'One Piece' is the 'Rio Poneglyph' itself. As the Rio Poneglyph is the history of the Hidden Century, hidden by the World Government, the owner of the Rio Poneglyph can coerce the world government, giving him absolute power.

Another theory is that in the opening credits, Gol D. Roger says he "left it all" (his treasure) in "One Piece". This could be a place and not the name of the treasure.