Mistress

Mistress was the female equivalent of the title Master. After regenerating into female form, the Master took the name, short for Mistress. (TV: Dark Water)

The K9 robots commonly addressed their female associates as Mistress. Leela, companion of the Fourth Doctor was Mistress to K9 Mark I. (TV: The Invisible Enemy etc.) Likewise, the first two incarnations of Romana were refered to as Mistress by K9 Mark II. (TV: The Ribos Operation, TV: The Creature from the Pit etc.) Sarah Jane Smith was the Mistress of K9 Mark III, who had been left to her by the Doctor, (TV: A Girl's Best Friend) and K9 Mark IV, who was left to her by the Tenth Doctor following the destruction of K9 Mark III. (TV: School Reunion) In addition, K9 Mark IV addressed Rani Chandra as "Mistress Rani". (TV: The Wedding of Sarah Jane Smith etc.)

Chip, a force grown clone, addressed the Lady Cassandra O'Brien.Δ17 as Mistress. (TV: New Earth)

Urak, leader of the Rani's Tetrap servants, addressed her as "Mistress Rani". (TV: Time and the Rani)

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)