What does “cis” mean?
A “cis” person is a person who was assigned a gender and sex at birth that they feel comfortable with. Typically, cis men are men who were assigned male at birth and feel that the words "man" and "male" accurately describe who they are. Likewise, cis women are women who were assigned female at birth and feel that the words "woman" and "female" accurately describe who they are. Generally, cis people feel comfortable with the aspects of their bodies that others inscribe with a sex and gender, and do not seek to modify their bodies in ways that would change how they or others place them in a sex category.
What is “cis” short for?
“Cis” can be short for “cissexual” or for “cisgender”. “Cissexual” and “cisgender” sometimes mean different things, but there is no single, agreed-upon definition for either word. Likewise, there's no single, agreed-upon definition for the words “transsexual” and “transgender”.
"Cis" is not an acronym or initialism. Spelling it "CIS" is incorrect. Contrary to popular belief, "cis" does not stand for "comfortable in skin." It is a Latin prefix, not an acronym made up of English words.
What's the difference between “cisgender” and “cissexual”?
Some people use “transgender” as a more genteel synonym for “transsexual”, and therefore also treat “cisgender” and “cissexual” as synonyms for each other. Other people treat “transsexual” as referring to a proper subset of people described by “transgender”. For example, some people use “transsexual” to refer to only those people who have modified their bodies in ways that don't meet their society's expectations for the gender they were assigned at birth, or who wish to modify their bodies in such ways.
Still other people treat “transsexual” and “transgender” as sets that intersect, with neither set containing the other. For example, some people use “transsexual” to refer to only those people who have never been able to meet expectations for the gender they were assigned at birth in a way that meets with social approval, using “transgender” for people who have the ability to meet expectations for different genders at different times.
So, analogously, for some people, “cissexual” people are a subset of “cisgender” people. For example, a person who violates their society's expectations for the gender they were assigned at birth, but has no desire to modify their body in any way, would be cissexual but not cisgender. But other people think that it's also possible to be cisgender but not cissexual. For example, some people identify themselves as transsexual (because they wish to modify their bodies in ways that violate society's expectations for the gender they were assigned at birth) and also cisgender (because socially, they have never been able to meet expectations for the gender they were assigned at birth).
It's best not to assume that “cis” stands for either “cisgender” or “cissexual”, and to ask for clarification when necessary. Likewise, it's also best not to assume that “trans” stands for either “transgender” or “transsexual”.
Where does “cis” come from?
In Latin, the prefix “cis” means “on the same side” and “trans” means “on the other side”. So, a cis person is one whose assigned sex at birth is on the same side as the sex they are. Likewise, a trans person is one whose assigned sex at birth is on a different side from the sex they are.
The German psychologist Volkmar Sigusch may have been the first person who use a term with the prefix “cis” to describe people who are completely or mostly comfortable with their assigned sex and gender, in his 1991 article “Die Transsexuellen und unser nosomorpher Blick” (“Transsexuals and our nosomorphic view”). Both Dana Leland Defosse and Carl Buijis have been credited with introducing the term “cisgender” online during the mid-1990s. More recently, Julia Serano has received some credit for popularizing “cis” in her 2007 book Whipping Girl. However, it's still possible that the term was introduced in the context of centering trans perspectives by an even earlier author.Alex Maestas. If you find it useful, consider donating directly to trans people in need.