A fairly well-known term which can mean one of two things:
1. Someone who was born as one gender but identifies as a member of the opposite sex, and wants to be considered as such.
2. A male/female that does not identify with the gender that they were born as and decides to undergo a sex change, to have his/her physical attributes and genitalia match with the gender they identify with.
Transsexual transitions can be either MTF (male to female) or FTM (Female to Male)
Ralph is a transsexual, he was born a woman and transitioned to a male, he even underwent a full sex change.