A T-girl is an individual who was born male who is either dressing as or has transitioned to a female.
More About T-Girl
T-girl is short for trans girl. The term is meant to be inclusive, which is to say that it includes those women who have transitioned, or plan on transitioning from male to female, while also including those males who simply wish to dress and live as women on the surface. The latter are more accurately referred to as crossdressers, but might also be referred to as T-girls.