The new Amazon Original Series, Transparent, about a middle-aged father who's transitioning into a woman, is just the latest cultural sign that the word "transgender" has gone mainstream. No doubt there have been transgender people—that is, those with a gender identity or gender expression that doesn't conform to their assigned birth sex—since there have been people at all, but the term itself wasn't coined until the 1970s. Popular confusion about its usage notwithstanding (for example, questions about the difference between "transgender" and "transsexual"), "transgender" is here, it's queer, and a lot of people have gotten used to it.

The situation is more complicated for "cisgender," coined in the 1990s to mean the opposite of "transgender." The "trans" in "transgender" comes from a Latin word meaning "on the other side of," and the "cis" in "cisgender" comes from a Latin word meaning "on this side of." "Cisgender" refers to people who feel there is a match between their assigned sex and the gender they feel themselves to be. You are cisgender if your birth certificate says you're male and you identify yourself as a man or if your birth certificate says you're female and you identify as a woman. Presumably you are also cisgender if you were born intersex (that is, with some combination of male and female reproductive parts) and identify as an intersex or androgynous person.

For a while, "cisgender" only appeared in academic journals. But now it's all over the Internet, and not just on blogs and sites of, by, and for transgender people. It's made it into online reference works like the Oxford Dictionaries. And since "cisgender" is one of the 56 options for gender identification on Facebook (along with "cis female," "cis male," "cis woman, "cis man," "cisgender woman," "cisgender man," and just plain "cis"), it has already achieved a kind of pop officialdom.

Even more auspiciously, Stephen Colbert referred to "cis-language" in a recent episode of The Colbert Report. On June 17, in a segment of the show called "The Word," the comedian announced that his racial identity is "ciswhite" because, as he put it, "I've always been comfortable with my birth-race." Colbert's announcement should be a big deal as far as disseminating "cis" goes. His 2005 coinage, "truthiness," became so popular that the American Dialect Society named it "The Word of the Year"—the term that best represented the times. In 2006 it was still going strong, as the Merriam-Webster Dictionary named it its "Word of the Year," too.

Still, "truthiness" never made into standard dictionaries of contemporary English. Neither have any of the "cis" words—at least not yet. Will "cisgender" go all the way into the English language, become mainstream, right alongside "transgender"?

Despite Facebook, even despite Colbert, "cisgender" may not last, let alone become a household word. And it's not because it started life as academic jargon. After all, "transsexual," the basis for "transgender," was first introduced in the American Journal of Psychotherapy. "Heterosexual" and "homosexual" were invented by neurologists in the late 19th century, to name what they considered to be opposing perversions ("hetersexuals" were deviants who enjoyed relations with the opposite sex for pleasure and not just for procreation). Since "transgender," "heterosexual," and "homosexual" have stuck despite their provenance, "cisgender" just might, too.

That said, "cisgender" sounds more improbable as a word than "transgender" ever did. While the prefix "trans" is familiar from hundreds of English words ("translate," "transport," "transcend"), "cis" occurs in only a handful, the least obscure of which is probably "cisalpine" ("on this side of the Alps"). Compared to "transgender," the meaning of "cisgender" isn't very transparent (so to speak). Research by Harvard linguist Steven Pinker has shown that neologisms with staying power can often be identified by their initial "unobstrusiveness" (see his 2007 bestseller, The Stuff of Thought: Language as a Window into Human Nature). Lexicographer Kerry Maxwell has since used a simpler (and more recent) word, "user-friendliness" to predict longevity. She says it helps if new words are easy to pronounce, easy to read, and easy to spell.

"Cisgender"? Not so much.

Of course, there's more at stake in the viability of "cisgender" than mere words. "Cisgender" suggests a commonality among transgender and non-transgender people, at a time when transgender people are struggling for recognition. It tells us that we all experience some kind of relationship between our bodies and our selves, whatever that relationship may be. And it reminds us that those who experience a "match" between their body and their selves have it a lot easier in our society than those who do not. To the extent that "cisgender" helps raise awareness of intolerance and injustice towards transgender people, it serves a crucial political purpose right now. Potentially, "cisgender" could help build consensus on transgender rights.

However, the politics of "cisgender" have already proven divisive. Perhaps the most surprising protests are coming from the left—from people, that is, who might otherwise be counted on to support the transgender movement. There are feminists who balk at the idea that cisgender women are privileged in relation to transgender women, who were born male. Among other potential benefits, such as "passing" as men in a patriarchal culture, transgender women don't have to worry about reproductive rights. The Huffington Post recently collected a grab-bag of very mixed reactions to "cisgender" from the gay community. It's clear that some gay men and lesbians see "cisgender" as a slur, a way of labeling them as elitists or conformists after all (i.e., as not "queer" enough). Some think "cisgender" validates the notion that there are two (and only two) genders, correlating with two (and only two) sexes, just as many are exploring non-binary gender identities, such as "genderqueer."

All of which brings us back to the problem of the word "cisgender" itself. Linguists agree that the survival of a neologism relies, above all, on whether it names a stable and coherent concept, an idea that will last. It's the uncertainty of the concept behind the word "cisgender," for now, that really hints at trouble.   

As the social and medical sciences tell us more and more about ourselves and one another, we may eventually settle on the right words, the true words or, at least, the ones that reflect the data. "Cisgender" may or may not end up being one of them. But maybe, in the long run, it doesn't matter. Maybe the impact of Facebook's 56 genders will one day prove to have had nothing to do with naming all the possibilities for gender identity, but rather with helping to make the idea of possibility, itself, official. Our vocabulary of gender and sex is in flux right now because our ideas about gender and sex are in flux, too.

So will "cisgender" be an upcoming "Word of the Year," the linguistic sign of our times? Or will we just forget about it?

Quite possibly, both.