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2.
1. Actual gender.

2. The rejection of the belief that your gender can change by merely *thinking* that you're a different gender.
A cisgender male = a male
A cisgender female = a female
by Salman Rushdie December 26, 2012
 
1.
Not transgender, that is, having a gender identity or gender role that society considers appropriate for the sex one was assigned at birth. The prefix cis- is pronounced like "sis".
John is cisgendered, he just holds to his role as a stereotypical male.
by freeyourmind July 26, 2004
 
3.
Essentially the opposite of transgender; whereas transgender people are those whose gender and assigned sex at birth don't match up, cisgender people are those whose gender and assigned sex at birth are the same.

This definition really shouldn't be hard to understand if you've got a vague grasp of what the word transgender means, but despite this, some cisgender people are under the impression that it's a slur; explaining to these people that it's about as much of a slur as the word heterosexual doesn't do anything to change their mind on this subject, so you can just safely dismiss these people as not very bright.
Jane is a cisgender woman; she was assigned female at birth, and identifies as female.

Sarah is a transgender woman; she was assigned male at birth, but identifies as female.

Bob is a cisgender man; he was assigned male at birth, and identifies as male. He objects to being called cisgender, stating: "I don't like that some tumblr tranny made up a term to describe me, it's insulting! I'm just normal!" He does not see the irony in objecting to cisgender but being perfectly a-ok with tranny, an actual slur.
by perezhiltonisafuckingasshole September 21, 2013