Highness

Highness (His/Her Highness, oral address Your Highness) was a form of address. In the context of royalty, the style was Royal Highness.

On Earth
The Fifth Doctor addressed Princess Villagra as "Your Highness". (TV: Four to Doomsday)

Princess Joanna of England was addressed as "Your Highness". Contemporaneously, William des Preaux addressed Saladin as "Your Highness". (TV: The Crusade)

Princess Marguerite, bride of Prince Henri of Navarre, was refered to as "Her Highness". (TV: The Massacre)

Tongzhi of China was refered to by George Litefoot as "His Highness". (TV: The Talons of Weng-Chiang)

In an alternate timeline, the King John Hart was mistakenly addressed as "Your Highness" by Rhys Williams, clarifying that was correctly refered to as Majesty while Highness was used for princes. (AUDIO: The Death of Captain Jack)

Elsewhere
In the Land of Fiction, Jamie McCrimmon addressed Rapunzel as "Your Highness". (TV: The Mind Robber)

The Draconian prince was addressed as "Highness" and "Royal Highness". (TV: Frontier in Space)

Graff Vynda-K was addressed as "Highness". (TV: The Ribos Operation)

Prince Reynart, first in line to the throne of Tara, was addressed as "Highness" and "Royal Highness". (TV: The Androids of Tara)

Princess Astra of Atrios was addressed as "Highness". (TV: The Armageddon Factor)

Highness was the leader of Guria. (AUDIO: The Human Factor)

Charlie Smith, the last Prince of Rhodia, was addressed as "Your Highness" by Andrea Quill. (TV: For Tonight We Might Die)

Behind the scenes

 * In NOTVALID: Death Comes to Time, Speedwell sarcastically addressed the Seventh Doctor as "Your Highness".