Gender

Gender was a sense of identity relating to whether one was male, female, masculine, feminine, or anywhere along a diverse spectrum of gender identities. (PROSE: Island of Death) Gender was a fluid concept, varying from person to person, culture to culture, and species to species. (COMIC: Ophiuchus)

As the Fifth Doctor explained:

Gender is a very fluid concept, Nyssa. For some people, more than others. A Time Lord even more so.Fifth Doctor, explaining gender

Amy Pond had met species which had only one gender, or three, or, in one case, seventy-two. (PROSE: Borrowed Time)

The Tenth Doctor told Rose Tyler that the Time Lords considered concepts like gender to be archaic. (AUDIO: The Sword of the Chevalier) The Twelfth Doctor explained to Bill Potts that the "most civilised civilisation in the universe" was billions of years beyond petty human obsession with gender and its associated stereotypes. However, she pointed out that they still called themselves Time Lords. (TV: World Enough and Time) Although "Time Lady" was also used, Missy considered the term "old-fashioned". (TV: Dark Water)

Later, while pretending to be the Doctor, Missy referred to Nardole and Bill as "Exposition and Comic Relief". When Nardole retorted that they weren't functions, she responded "Darling, those were genders." (TV: World Enough and Time)