Why You Should Say "Cis"
The word 'cis' was coined by trans people in order to describe people who were not trans, and this was done because no word existed to describe them. Though one could say 'non-trans,' this defines people as what they are not, and is ultimately unwieldy. The other option would be something like "normal," "real" or "biological."
Cis people did not have a word to describe themselves as a group, prior to the invention of the word "cis," besides words like "normal," "real," or "biological." All of these words are transphobic and inappropriate. Trans people are not abnormal, less real, or non-biological. Prior to the invention of the word/prefix "cis," there was no word for non-trans people that did not stigmatize trans people in some way.
Trans people used some other words to describe non-trans people, prior to the emergence of the word "cis" into mainstream usage. These include "genetic (girl/boy/man/woman)" and "natal (girl/boy/man/woman)." Neither of these formulations are particularly viable. The genetic prefix puts the focus on bodies and chromosomes, neither of which are a helpful focus for meaningful trans liberation. In order to use genetic as a type of classification system for trans/non-trans people, we would have to assert that some genetic configurations create womanhood or manhood naturally, while other men and woman must be created in another way.
A more liberated perspective asserts that genes operate independently of internal gender identity; that is, an XX set of chromosomes does not inherently result in a female, feminine, or "woman" identity, and visa versa. This is our perspective.
As trans people, we have decided that our liberation is best served by emphasizing gender identity as being much more important than genetic code or types of chromosomes. We have emphasized the human, social experience of gender, rather than the biological underpinnings of chromosomal sex. This is a more humane and human perspective, and as time goes on, terms like "genetic girl" or "natal female" are falling to the wayside quickly.
Additionally, there are other problems with terms like "real," "biological," "natal" or "genetic" to describe how one came to one's gender. To use terms like "genetic male" is confusing in terms of genetic diversity-- plenty of trans-identified and non-trans-identified people, are intersex (see the post and comments here for more on the intersex/trans distinction.). Whether someone identifies as male, female or another non-binary identity stands apart from their genetics and their sex as assigned at birth.
And what's more, many people do not identify as male or female, man or woman-- such as genderqueer, gender variant, two-spirit, non-binary, third gender people and many others. The natal/genetic identity prefix was only meant to encompass the distinction between a binary-identified (exclusively man or woman identified) transitioned or transitioning trans person, and a binary-identified cis person. It comes from an earlier time when non-binary gendered people were still very much hidden and not understood.
Some people are concerned that cis is not well-defined, and shouldn't be used-- or that it sets up a new binary of sorts. Simply put, cis means someone who was assigned a certain sex at birth, raised in the socially corresponding gender, and continues to identify as such. A person who is assigned female at birth (generally, based on the doctors', parents' or nurses' evaluation of that baby's genitalia), raised as a girl, and identifies as exclusively as a girl, woman, or female, is cis. A person who is assigned male at birth (based on the doctors', parents' or nurses' evaluation of that baby's genitalia, and possibly their chromosomes), raised as a boy, and identifies as exclusively as a boy, man, or male, is cis.
Is it just that simple? Nope, sorry. Life is complicated. Here's the deal:
Some people are not clearly cis or trans. For example, some people do not feel comfortable claiming that they are trans, but also do not feel that they are cis. They occupy a middle area. Perhaps they have some feelings of identifying with the sex other than the one assigned them at birth, but they are not sure how strong those feelings are, how much they affect their life, or what they wish to do about them.
While these individuals deserve the same compassion, support, and where possible, access to trans spaces that trans people do, this does not negate the usefulness of the categories "trans" and "cis" any more than the color green negates the usefulness of the categories "blue" and "yellow." Complexity will always exist, but that doesn't mean that words or concepts are useless. It's okay to not be either cis or trans, just as it's okay to not be either man or woman. It means that when conversations about privilege are happening, people who are neither cis nor trans will need to parse those conversations more carefully and more individually. Some things may apply to them, some not.
Additionally, some people who have fully transitioned no longer identify with the word "trans" or "transgender." They do not experience themselves as part of the conceptual meaning of "trans" (to go across, or to transcend, or to be on the other side of). They also may not see themselves as cis. Additionally, some people, regardless of their transition status, do not like the word "trans" and prefer "affirmed male," or "affirmed female." All of these people should be supported in their chosen use of language for themselves, but this also does not negate the usefulness of terms like "trans" and "cis" in many situations.
Some feel that words like "trans" or "cis" just divide us further. "We are all just human beings!"
Words like trans and cis are necessary, not to divide us, but because we already are divided. Unfortunately, trans people are divided from cis people by our life experiences in critical areas like access to jobs, medical treatment, social acceptance, safe housing, freedom from violence, and the ability to live in, and be seen as, the gender(s) with which we identify. (Other kinds of oppression, such as being a person of color, being disabled, &c. play a great role in producing inequalities as well. When compounded with being trans, disparities between populations are even larger and systemic violence is also more extreme.)
Trans people, and often trans women in particular, face serious disparities (inequalities) in mental and physical health and economic well-being. A wealth of statistics now verifies that trans people face serious barriers in many aspects of life. One in five transgender people in the U.S. have been refused a home or apartment, and more than one in ten have been evicted, because of their gender identity (source). 41% of trans people have attempted suicide (source). Trans people are medically insured at a lower rate than the general population, and almost 50 percent of transgender respondents have postponed medically necessary care for financial reasons or because they were afraid of encountering discrimination from providers (source).
Without words like "trans" and "cis" we cannot discuss these disparities as effectively, and without being able to discuss them, we cannot work for change. If we cannot work for change, we are powerless to make the lives of trans people better. Our allies know that words like "trans"and "cis" are tools to fight inequality and uncover systemic violence and discrimination against trans people. Language is one of the best tools that we have-- if we can't say it, we can't think it, and if we can't think it, we can't even begin to act.
Some people feel that they simply don't like having a label or additional identity. It is understandable that some people do not want to be labeled--either as trans or as cis. It is important to understand that trans and cis are not identities. That is, they are not usually thought of as gender identities, the way that "woman," "male," or "genderqueer" are.
Instead, they refer to states of being and often to gender histories. A person may identify as a woman, and also be cis. In this case, woman is her gender identity, and cis describes the way her womanhood is culturally situated. Because she is cis (assigned female at birth, raised as a girl, and identifies as a woman) it is likely that she experiences privileges that trans women do not, depending on her culture and how that culture views gender. For example, she may be able to use the women's restroom in public areas without fear of being kicked out by security personnel. This is not because she is a woman, but because she is a cis woman.
The word "cis" is our best way to describe the social position this woman occupies that allows her to access this privilege. It gives us a way to ask: If two women share the same race, are abled or disabled in the same ways, come from the same economic background, and so forth, why would one woman possess the privilege to freely access the women's room, and the other would do so only with justifiable fear and apprehension? Our simple answer, in this case, is that one is trans and the other is cis. These words don't label gender, they label the social position that these gender identities occupy. They give us power by naming a type of inequality in our society.