Mistress

Missy. (TV: In the Forest of the Night)

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

K9 addressed Leela, (TV: The Invisible Enemy, etc.) Romana (TV: The Ribos Operation, etc.) and Sarah Jane Smith (TV: A Girl's Best Friend) as "mistress".

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