Original: Princess of Orange were women married to the Sovereign Princes of Orange during their reigns from 1171 to 1815, and the heirs to the Dutch throne, who were titled Prince of Orange, since 1815.
Simplified: Princess of Orange is the title (name) used for the women who were married to the Ruling Princes of Orange in the time between 1171 to 1815. Since 1815, it was also used for the wives of the heirs to the Dutch throne, who were titled Prince of Orange.