Cisgender (often shortened to cis) is the opposite of transgender or cisn’t, meaning someone who always identifies wholly and exclusively as the gender they were assigned at birth. The term is not an identity by itself, rather it is a term used primarily by the transgender community as a way to describe those who are not trans. It describes men who were assigned male at birth and women who were assigned female at birth. Some intersex people who are non-binary may think of themselves as cisgender.
Cisgender vs. Cissexual
Cissexual or cissex, is a term based off transsexual/transsex. In some contexts, it can be useful to distinguish between cisgender and cissexual, along with distinguishing between transgender and transsexual. Cissexual can refer to someone who has not changed their bodies through medical interventions, such as hormones and/or surgeries, and does not seek to medically change their body.
This distinction can be useful as it's possible to be transgender and cissexual, meaning one identifies as another gender, but does not wish to medically transition. It is also possible to be cisgender and transsexual. For example, some drag artists and other gender non-conforming people may identify as cisgender, but still seek aspects of a gender transition.
The cisgender flag was created by DeviantArt user FlagsforCisHets on April 4, 2016. No additional information is given about the flag.
Two other flags were created on the same day by the same user. The pink and blue flag is said to have been created as a heterosexual flag by arianod. However, it was posted with the title "Straight/Cisgender" flag. The flag with the white heart was given the same title. It is unclear if these flags were intended as cisgender flags, as heterosexual flags, or as a cisgender heterosexual flags. No additional information is given for either of them.
A fourth flag was created by LGBTA user OMeera on June 8th, 2021. The grayscale cisgender flag is depicted using black, and variations of gray. The heterosexual flag uses black and white (grayscale) to represent "lack of color", as opposed to the LGBT+ flag, which is colorful. Likewise, being Cisgender is the opposite of being Transgender,
"Cis" comes from the Latin prefix meaning "on the same side", as in the cis-trans distinction in chemistry. The origin of the term in reference to gender is credited to Carl Buijs who proposed the term in 1995, and although the word had been used on the internet before 1995, Carl Buijs is the first recorded source to define the term. It's possible that the term was independently coined elsewhere.