Cisgender is a term for someone who has a gender identity that aligns with what they were assigned at birth. The term was created for referring to "non-transgender" people without alienating transgender people. For example, if the doctor announces a baby as being a girl, and she is fine with being a girl, then she is cisgender.
Cisgender may sometimes be referred to as cissexual (corresponding to transsexual, not to be confused with a sexual orientation), or shortened to cis.