gender

First recorded in 1300–50; Middle English, from Middle French gendre, genre, from Latin gener- (stem of genus ) “kind, sort”

Although it is possible to define gender as “sex,” indicating that the term can be used when differentiating male creatures from female ones biologically, the concept of gender, a word primarily applied to human beings, has additional connotations—more rich and more amorphous—having to do with general behavior, social interactions, and most importantly, one's fundamental sense of self.
Until recently, most people assumed that acknowledging one's gender, or sex, was easy. You just checked the appropriate box on a standard form, choosing either “male” or “female,” according to the gender you had been assigned at birth based on visible anatomical evidence. But some people's internal sense of who they are does not correspond with their assigned gender. And in fact, we now recognize that a complex spectrum between male and female exists not only mentally, psychologically, and behaviorally, but also anatomically; there have always been intersex people. The conflation of gender with sex, though historically common, is now often criticized because it is seen by some to be insensitive or dehumanizing.
Gender identity is complicated. Some people, perhaps most, do not question their assigned gender. But others perceive themselves as belonging to the opposite sex. Still others, some of whom identify themselves as genderqueer see themselves as neither male nor female, or perhaps as both, or as rotating between genders, or even as not belonging to any gender categorization at all.
Those who clearly see themselves as the opposite sex may or may not want to transition to it in some measure. Of those who do, some may complete that transition, but others may be happy to stop partway on a path that can include dressing and living as the opposite sex, although the desire to crossdress can exist quite apart from issues of gender identity. Somewhere along the transitional path people may want to change their given names and adopt linguistic terms of their own choosing, including a variety of pronouns, as designations of themselves and others. Some will have hormone treatments and opt for various kinds of surgery—perhaps facial, perhaps on their bodies, perhaps ultimately including sex “reassignment” surgery (genital reconstruction). At any point, they may welcome or reject a “transgender” label.
This array of life experiences has resulted in a veritable explosion of new, or newly adapted, vocabulary. Particularly striking and useful is the word cis or prefix cis- as in cis male, cis female, and cisgender, designating those whose sense of self matches their assigned gender. Using cis is a way to refer to these individuals without implying that “cis” people are the norm and all others a deviation from “normal.” It is notable that choices of gender beyond male and female now appear on social media sites. Clearly, gender is no longer a simple binary concept.

1300–50; Middle English gendren, genderen <Middle French gendrer <Latin generāre to beget, derivative of genus gender¹, genus