Short for cisgender, this term is used to describe a person whose gender at birth matches their gender identity.
This term is the opposite of transgender, which describes someone who has a gender identity that does NOT match their gender at birth.
Cis is a colloquial term and is even sometimes used in a derogatory fashion.
Even though Ryan is a cis, he often goes to LBGT meetings in his local area to support his friends who are transgender.