Personne dont l'identité de genre correspond au sexe qui lui a été assigné à la naissance.
Avant l'existence du mot « cisgenre », il n'y avait pas de terme neutre pour les personnes dont l'identité de genre correspondait au sexe assigné à la naissance : on les disait simplement « normales », ce qui présentait implicitement les personnes trans comme anormales. Les militant·es trans ont fait valoir qu'un terme parallèle et neutre était nécessaire. Le mot est aujourd'hui standard dans les contextes universitaires, médicaux et communautaires.
Myth: cisgender is a slur
It is a descriptive adjective with a Latin prefix, coined in academic writing and now in every major English dictionary. It has no history of being used to demean anyone, and it does not name anything shameful. The reaction is usually less about the word and more about the discomfort of being categorised at all, which is an experience trans people have every day.
Myth: nobody asked to be called this
Nobody asked to be called heterosexual either. Descriptive categories are not opt-in. If the objection is that a word implies trans identity is equally normal, that is not a misreading of the word, it is what the word is for.
Myth: cisgender means conforming to gender roles
It does not. Cisgender is about identity matching assignment, and nothing else. Butch cis lesbians, feminine cis men, and cis people with no interest in gendered expectations are all cisgender.
Myth: you cannot be cisgender and intersex
You can. An intersex person whose gender identity matches the sex they were assigned at birth may describe themselves as cisgender, though many intersex people find neither cis nor trans fits well and prefer to leave the question alone. Follow the person's own usage.
Myth: cisgender is an insult when used in arguments
A neutral word can be used rudely, like any other. "Cis people should listen here" is a claim about a conversation, not an insult. If the word is doing work you dislike, the disagreement is with the claim, not the vocabulary.