Not sure about the older definitions of sissy, but in the non-CDing world it is a pejorative term for feminine men. In the CDing world it is a type of fetish, for example dressing like a little girl in petticoats, or a sissy maid or other submissive female.
"Transvestite" is the latin form of "crossdresser" and both terms are interchangeable. For some reason, many crossdressers in the US believe the term is applied to a fetish crossdresser, but in truth, there are fetish crossdressers/transvestites and there are non-fetish crossdressers/transvestites. "Transvestite" is used quite a bit overseas. In most academic research, the term "transvestite" is used to describe men who wear women's clothing.
