What Does Cisgender Mean?
Cisgender refers to people who experience and present their gender in a way that's aligned with the sex of their body. It contrasts with transgender, which refers to people who experience their gender as different from the physical sex they were assigned at birth. Generally, transgender folks take various steps to bring them into closer alignment, such as wearing clothes of the gender they feel themselves to be, surgery, taking hormones, and having their legal name changed.
The word has been in use since at least 1994, although it has become more well-known since Julia Serrano’s book Whipping Girl came out. Cisgender is an important word to use because it allows us to reconsider the idea that being cisgender is “normal.” When we assume that man = cisgender man unless we use the term transgender, we reinforce the idea that cisgender people are normal and transgender people aren’t. Of course, being cisgender is more common, but when we use language that reinforces the idea that more common equals "normal," we marginalize people who are well within the range of diversity that exists in the world.
Cisgender is also a preferred term over bio-guy or bio-girl, which were in use for a while, because it shifts the focus from biology to gender. Similarly, the term genetic man (or woman) isn’t really useful since most people haven’t been genetically tested and there’s no guarantee that a person's appearance or identity will be caused by or reflected in their genetics.
Cisgender is a really useful word and it serves a valuable purpose. Use it, pass it on, and help it spread.