Believing that a person identifies with a specific sexual gender, other than that based on their physical attributes, or genitalia.
Where a person born as what society believes is one sex, based on the assigned genitalia, the person identifies with the other gender more, due to the wiring of their brain.
In short, a transsexual man, will believe they are more a woman, than a man despite having a penis, and a transsexual woman, will believe that they feel more like a man, than a female, despite the lack of a penis.
Western culture is really just beginning to fathom the depths of Transsexualism, though it has been noted in most Native American Tribes, from early on.
Natives, or First Nation's People refer to transsexuals as 'two spirited' in that they carry the traits of both genders within, and are revered in their culture, holding various high offices, within the tribe, such as medicine men.
Such a condition, or belief, is still considered 'wrong' in most of Western Culture, and the use of Sexual Reassignment Surgery is controversial at best. Usually these people are discriminated against, due to religious beliefs or cultural values of society.
Many 'transsexuals' belief that their sexual gender is hard wired, within the brain's circuitry, prior to birth, and that it occurs prior to the development of the genitals.
Society determines the 'gender' based on the external genitals such as a Vagina, or Penis.
However, Transsexuals, identify more with the sex, of which they are not physically equipped as. They instead believe or assert that their brain has been coded, or hard wired, to be more in tune with the other gender, that they have no control over that pattern, or inner feeling.
Transsexuals are not defined as being Homosexual, unless they desire to engage sexually with those of the same sex, that they believe they are. In other words, a Transmale may desire sex with a man, but while they are biologically a male, it is their internal desire of being a female, that sets the orientation. This means that in theory, they are not homosexual, but heterosexual.
Practice (Associated Acts): Transmales - refers to someone who is born as a woman, physically (biological), but identifies with being a male.
Transwoman - refers to someone who is born as a man, physically (biological), but identifies with being a female.
Noteworthy: To fully understand the term, it should be noted that 'trans' refers to crossing over and that a person who appears as one gender, wishes to cross over to being the other, and is not the same concept as being homosexual.
Internally, they believe they are one sex, desiring to engage with the opposite sex. This makes them heterosexual, not homosexual.
Unless they obtain a sexual reassignment surgery, they may seek out other's of the same biological sex that they are, and prefer that sexual relation, but internally, they are experiencing it as being the opposite sex. Some head doctors would not consider their actions as being homosexual, due to that internalization.
Many will assume that those who prefer to dress as the opposite sex, to them, are transsexuals, which isn't always the case.
There is a difference, in that Transvestites (Cross Dressers) do so because they enjoy the act of dressing up, prefer the feel of the other gender's clothing, not because they believe they are that 'other' gender.
Transsexuals undergoing Sexual Reassignment Surgery will dress in the sex they are crossing over to, but that is for other reasons. It is done, to acclimatize them to their new life, as the opposite gender to their present biological one.