Someone who identifies with their assigned gender at birth. That’s literally it, everyone chill out.

Commonly used in text messages or social media as a short form for Very Sound Advice.

an adjective for someone whose gender corresponds to their assigned sex

Cisgender (often abbreviated to simply cis) is a term for people whose experiences of their own gender agree with the sex they were assigned at birth. Cisgender may also be defined as those who have "a gender identity or perform a gender role society considers appropriate for one's sex."

When a person identifies as the sex they were assigned at birth.

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".

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.