Your Royal Highness or Imperial Highness It depends on the country and rank of the Princess because some princesses have the style "Highness" insetad of Royal or Imperial Highness.
Princess Anne is the firstborn daughter of the English monarch, and so is the current Princess Royal.
Princess Anne has been Princess Royal since 1987.
Princess Anne, the Princess Royal, is married to Sir Timothy Laurence.
The usual title granted is Princess, though this can have prefixes or suffixes, as in Princess Royal. The formal form of address is your Royal Highness. Less formally Ma'am. The title of Duchess is usually granted also
Princess Royal Stakes was created in 1946.
1. female given name 2. Title for a princess
The Princess Royal was King George VI's sister Princess Mary, who died in 1965
Call princesses by the proper title. As these are generally the second highest ranking royals, be sure to address them properly. Princes and princesses must be addressed differently. For a prince, say, "His Royal Highness, Prince of (Name of Country). For a princess, say, "Her Royal Highness, Princess of (Name of Country).
Princess of Wales's Royal Regiment was created in 1992.
The Princess Royal Trust for Carers was created in 1991.
LMS Princess Royal Class was created in 1933.