Imperiatrix 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, took the title Imperiatrix as she exercised more power than her position allowed. The High Council deposed her and had her dispersed from history. (AUDIO: Imperiatrix, Lies)
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 Inquisitor Darkel's insurrection, Romana held the position of Imperiatrix, in a wake of a civil war, she kept the Nekkistani Emperor and threatened to destroy the Nekkistani race. (AUDIO: Imperiatrix)