A queen regnant is a female monarch, equivalent in rank, title and position to a king. She reigns suo jure over a realm known as a kingdom; as opposed to a queen consort, who is married to a reigning king; or a queen regent, who is the guardian of a child monarch and rules pro tempore in the child's stead or instead of her husband who is absent from the realm, be it de jure in sharing power or de facto in ruling alone. A queen regnant is sometimes called a woman king. A princess regnant is a female monarch who reigns suo jure over a principality; an empress regnant is a female monarch who reigns suo jure over an empire.

Irene of Athens, surname Sarantapechaena, was Byzantine empress consort to Emperor Leo IV from 775 to 780, regent during the childhood of their son Constantine VI from 780 until 790, co-ruler from 792 until 797, and finally empress regnant and sole ruler of the Eastern Roman Empire from 797 to 802. A member of the politically prominent Sarantapechos family, she was selected as Leo IV's bride for unknown reasons in 768. Even though her husband was an iconoclast, she harbored iconophile sympathies. During her rule as regent, she called the Second Council of Nicaea in 787, which condemned iconoclasm as heretical and brought an end to the first iconoclast period (730–787). Her public figure was very polarizing during her 5 year reign, as most saw it as wrong for a woman to rule solely. Her reign as such made her the first ever empress regnant, ruling in her own right, in Roman and Byzantine imperial history.