Royal

A Royal is a relative, or a member of the ruler of a kingdom family. The term is generally heriterial at birth or by marriage, however, the ruler family can change via hostile takeover or the demise of all possible heirs. The royal family, or "Monarchy", consist of a King patriarch, a Queen matriarch, and a Prince and/or a Princess heir.

Royalty in One Piece
Most kingdoms in the world of "One Piece" are ruled by a royal family of sort. Most of the rulers are monarch, however some ruler are called by other names while retaining their status and privilege: "Empress" in Amazon Lily, "God" in Skypiea, "Duke" or "Master" is Zou, "Lord" in Wano for example. The patriarch of Kamabakka is referred to as "Queen" due to the culture of the kingdom.

The titles can be revoked with the replacement of the ruler by means of force, rebellion of the people, the monarchy abandoning their kingdoms or by the the members of the royal family. Ruler who have abandoned their kingdoms and their descendants may claim to still have ownership of the country years after they left.

Every four years a meeting of the current kings is held in Mariejois, called Reverie.

Some kingdom has a group of people whose job is to guard the royal family members.

Royals
During the Void Century, twenty unnamed kings join forces to take down the Great Kingdom and nineteen of them later left their countries to go live in Mariejois, where they changed their titles to "Celestial Dragons".