Cisgender (often shortened to cis) is the opposite of transgender, meaning someone who identifies with 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.
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, 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.