Dressing up in costume, where one takes on the identity of a different gender.
Coined in the Anime and Hentai world, this is where the 'characters' are dressed up in clothing to take on a different gender appearance. It is similar to crossplay.
It had a negative stigma in society, and some of the clothing styles were designed to indicate a sexual desire, preference. In North American culture, this is more commonly known as 'sex role playing' rather than 'cosplay'
Originated more in the anime field than anywhere else, that has developed into more 'live' events.
Japanese Sex Clubs, known as 'image clubs' have staff dress up in costumes such as nurses, police, military, or in some cases, as popular anime characters.
This involves the wearing of intricate costumes, to create the full illusion of the person being of a different gender, unlike cross dressing, or genderfuck.
It is considered an art form in Japan, and is becoming more popular in live action such as movies and shows.
Practice (Associated Acts): A form of 'cosplay' is when one dresses up in authentic garb of a period, or from a favorite science fiction, historical period. This is basically termed 'role playing'
The line is blurred but when one dresses up in costumer, for the purpose of becoming sexually aroused, or to create sexual arousal, it appears to become less 'cosplay' than simple role playing.