Princess

A Princess is a female heir to the throne. She is usually a daughter of a King and/or Queen and is in a position where she legitimately could inherit her country's throne.

Overview
As a woman, in most real world cultures, a princess is the last person within the immediate family who is in line for the throne. If the King dies or retires without having a male heir, the princess will become the ruler and take over as queen. She usually loses her rule if she acquires a husband, since being a man, he takes over as the ruler. She can also become queen if she marries a man who either is or will become king himself.

Princesses, because of their status, can be subjected to a number of diplomatic solutions. A new king who has conquered a country may marry the eldest princess of the previous ruling family. This finalizes his right to the kingdom both with the people of the country and the family of the princess, as the family still has its ties to leadership. This also ensures all possible sons the princess may have will be his heirs and not be rivals to his own kingship. Another scenario is granting her hand in marriage as part of a peace treaty or an offering of alliance to another opposing force.

One Piece
Currently, only four princesses made an appearance in One Piece. The most notable princess is Nefertari Vivi of Alabasta who traveled with the Straw Hat Pirates during the Baroque Works Saga. Vivi does not follow the real world tradition of a princess in many ways, in the sense she can be more like a commoner at times than royalty. The second prominent princess is Shirahoshi of Ryugu Kingdom, who is also considered to be the Mermaid Princess, a legendary figure who can summon Sea Kings. Manshelly is the princess of Tontatta Kingdom, and accordingly, she is spoiled, cruel and selfish, though still a comrade for her people to want to save her. Hatchan met the Goldfish Princess of Goldfish Empire during his seafloor stroll.

Princess as an Epithet
The term "princess" is also used as an epithet, such as Perona the "Ghost Princess" and Boa Hancock the "Snake Princess".