"Cis" is short for "cisgender," referring to a person whose gender identity matches their biological sex. The Latin prefix "cis-" means "on this side of;" a cisgender individual's gender identity is on the same "side" as his or her sex.
Cisgender is the opposite of "transgender," in which a person's gender identity and sex do not correspond. The prefix "trans-" means "on the other side of."
In addition, cis can also refer to the structure of a molecule. In some cases, two molecules can have the same chemical formula but different geometrical structures or "isomers." A cis-structured isomer has two atoms or chemical groups located on the same side of a carbon-carbon bond, while a trans structure has the same two atoms located on opposite sides of the bond.