Master

Master was a title which was claimed by the renegade Time Lord known as the Master, to the point that he asserted it as his name. (TV: Utopia, TV: The Sound of Drums) After regenerating into female form, the Master took the name, short for Mistress. (TV: Dark Water)

The K9 robots commonly addressed their male associates as Master beginning with Professor Frederick Marius, the creator of the original K9 Mark I. When Marius left K9 to the Fourth Doctor, K9 knew him as Master or "Doctor Master", (TV: The Invisible Enemy, The Sun Makers etc.) as did K9 Mark II. (TV: The Ribos Operation, The Pirate Planet etc.) K9 Mark III recognised the Tenth Doctor as his Master, (TV: School Reunion) as did K9 Mark IV. (TV: Journey's End) In addition, K9 Mark IV addressed Clyde Langer as "Master Clyde" (TV: The Mad Woman in the Attic etc.) and Luke Smith as "Master Luke". (TV: The Wedding of Sarah Jane Smith etc.) K9 Mark 2, the regenerated form of K9 Mark I, exclusively addressed Starkey as "Young Master". (TV: Regeneration)

Mister Cole was the master of the Hazel Street Workhouse. (TV: The Next Doctor)

As an adjective
K9 Mark 2 noted that the all-female Ukkan librarians had completely mastered, or rather "mistressed", the art of archiving. (TV: Lost Library of Ukko)