Cissexual is a term for a person whose gender presentation and identification matches what is conventionally thought of as their "birth gender" or "biological gender". The term is formed by analogy with "transsexual": since "trans" is the opposite of "cis", a person who is not "transsexual" is "cissexual". The terms "cisgender" and "cis" are also used, and "cishet" is used to describe a person who is both cissexxual and heterosexual.
As can be imagined, the term itself is not accepted by everyone. The term was coined by transsexual people and their allies to highlight the fact that being cissexual, once a nameless state taken to be "normal", is itself just another choice. Given the fact that many people are antipathetic to the transsexual, the usage of "Cissexual" is usually a shibboleth.