Related terms include cishet, cisnormativity, and cissexism.
"I'm am cisgender: although I'm a tomboy, I've been identified as a girl my whole life and I have always considered myself to be a woman."
Often shortened to "cis," cisgender is a term used frequently in the intersectional, feminist, LGBT+, and transgender communities. It is an adjective used to describe someone whose gender identity matches their body and the gender assigned to them at birth-- in other words, someone who is not transgender, nonbinary, or intersex. A cisgender person might be an AFAB who identifies as female or an AMAB who identifies as male.
Cisgender individuals are not necessarily people who express or embody only gender-typical or gender-stereotyped behaviors. A cisgender woman, for example, might have a masculine gender presentation (i.e. butch) but still identify herself as a woman. Likewise, AMABs who are gay, queer, bisexual, pansexual, effeminate, sissy, or atypically masculine are still cisgender men so long as they identify themselves as men.
The word cisgender was first used in 1995 in the peer-reviewed article "Transsexueller Wunsch und Zissexuelle Abwehr," or "Transsexual Wish and Cissexual Defense," by German sociologist Volkmar Sigusch. It was developed as a complement to transgender, since cis-, meaning "on this side of," is used in Latin as an antonym to "trans-, meaning across from."