Imperiatrix

You may be looking for the Imperator.

Imperator or Imperiatrix of Gallifrey was a title held by the leader of the Time Lords during times when the High Council was disbanded and the leader ruled in a more dictatorial manner. Imperiatrix personal guards were known as the Imperial Guard.

Pandora, the first female ruler after Rassilon, (AUDIO: Imperiatrix, Lies) took the title Imperiatrix as she exercised more power than her position allowed. The High Council deposed her and sentenced her to molecular dispersal. (AUDIO: Imperiatrix, Lies)

Upon his election to the Presidency, Morbius similarly overthrew the High Council (TV: The Brain of Morbius) and declared himself the Imperator. He was later betrayed to the High Council by his former ally Thessalia (PROSE: The Book of the War) and condemned to molecular dispersal. (TV: The Brain of Morbius)

In one possible reality caused by the influx of anti-time, Romana II held the position of Imperiatrix of the empire of Zagreus, keeping the Dalek Emperor captive, and threatening the Dalek race with destruction. (AUDIO: Neverland) In another possible reality caused by Pandora's influence and Darkel's insurrection, Romana held the position of Imperiatrix in a wake of a civil war, and she kept the Nekkistani Emperor and threatened to destroy the Nekkistani race.

Madam President Romana temporarily took the title to prevent a newly resurrected Pandora from usurping her rule. (AUDIO: Imperiatrix)

Other references

The human Emperor Ludens Nimrod Kendrick Cord Longstaff XLI claimed to be the Imperator of known space. (TV: Nightmare in Silver)

The leader of the Fifth Axis used the title "imperator". (PROSE: The Glass Prison)