From Wikipedia:

The word cisgender (often shortened to cis; sometimes cissexual) describes a person whose gender identity corresponds to their sex assigned at birth, i.e., someone who is not transgender.[1][2][3] The prefix cis- is Latin and means on this side of. The term cisgender was coined in 1994 as an antonym to transgender, and entered into dictionaries starting in 2015 as a result of changes in social discourse about gender.[4][5]
Related concepts are cisnormativity (the presumption that cisgender identity is preferred or normal) and cissexism (bias or prejudice favoring cisgender people).