Dressing up in costume, to portray a different gender or sex of the person wearing the costumer.
Very similar to Cosplay, but more of a western world version. This involves the wearing of elaborate costumes, clothing, to hide the gender of the person, or to portray a different gender.
Female to Male (FtM) is far more common than Male to Female (MtF) due to several factors, mostly based on how society views such an act. In most cultures, Male to Female will generate a response that the person is exhibiting homosexual desires, or transsexual feelings.
This is generally not the case, but the negatives tend to make men more withdrawn, less willing to dress up as a female character. This stigma is not as noticed when the person is a female portraying a male.
In some circles, where women dress as men, it is thought that it is 'penis envy' or of having 'lesbian' tendencies.
Wearing of costumes, clothing, that are intricate in detail, even beneath the outer layer visible to the audience or public. Generally taken from Anime, this is the portrayal of a different gender.
Practice (Associated Acts): Passing - This is where a person will actually live the lifestyle associated with the gender they 'assume' by the costume, in an effort to hide their true gender.
Mostly this is females dressing as males.
Noteworthy: In some respect, this is like being totally wrapped up in a character, such as Trekkies are in the Sci Fi show Star Trek. They dress up as their favorite character, and could be considered 'cosplay' or 'crossplay' in loose terms.
Crossplay / Cosplay is baaed generally on Anime Characters, and in Crossplay, the Sailor Moon is a classic for MtF crossplay fans. It also created much of the negative stereotype associated for MtF, primarily due to the costumes themselves.
Mainly, this is a form of bending the perception of the roles of gender, as well as fandom, the appreciation of particular characters.