One Piece Encyclopedia

Impel Down It is an underground prision located in the governmental island, Enies Lobby. Here it is where the most dangerous criminals of the world are sent.
Island Kujira A type of whale from the "West Blue". They can reach the size of an island, hence their name. They usually travel in whale groups or gam.
Isshi-20 When Wapol exiled all the doctors from Drum, he choose a group of selected doctors for his personal use, that group is called the "Isshi-20". All citizen who needed be taken care of doctor must go to Wapol and suplicate that they cure him.
Ittouhei "First class private recruit" - A person recruited by marines to exert as soldier.
Ittouryuu Ittouryuu is a sword discipline that consists in using only one sword.