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 should be known that “cis” people are not categorized as being LGBTA+, unless they, of course, are not straight. This would mean that if someone is born a female, they will identify wholly as female, and vice versa.
Etymology
"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[1], 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.