Has the world in One Piece's world map been fully defined?
One Piece world I think is more or less explored.

What Nami's dream is not to explore the unknown reaches of the world, but rather to map them.

She wants to draw a map of the world which includes all the 5 oceans, redline and maybe even the sky islands.

The reason why finding the one piece is important for her is because the One Piece is supposed to be in Raftel.

Though Gol D Roger and his crew have visited it, the location of the Raftel is unknown to the world and is not charted yet.

So Nami's dream is to draw a map of the world which has everything including the location of Raftel, Sky islands and all the other places for which proper maps do not exist.