Transsexual

A transsexual (sometimes transexual) person establishes a permanent identity with the opposite gender to their assigned (usually at birth) sex. Transsexual men and women make or desire to make a transition from their birth sex to that of the opposite sex, with some type of medical alteration (gender reassignment therapy) to their body.

The stereotypical explanation is of a "woman trapped in a man's body" or vice versa, although many in the transsexual community reject this formulation.