"Cis" or "cisgender" is a term which means that someone has the same gender identity as the gender they were assigned at birth - it means that the person is not transgender.
The word comes from a Latin prefix meaning "on the side of", while the prefix "trans" means "on the other side of". These prefixes also used in biology, geography, chemistry and genetics.
The first use of the "cis" prefix referring to gender comes from a 1914 text by German physician and sexologist Ernst Burchard. He used "Cisvestitismus" to describe the inclination to wear clothing which conforms with a person's gender - the opposite of "Transvestitismus" (transvestism), the inclination to wear gender non-conforming clothes.