The dressing in clothes traditionally worn by the other gender.
To deliberately wear clothing to the other gender, in order to elicit a response. To play with people's minds, to be outrageous and rebellious.
It is also more of a political statement, to suggest that people should be comfortable in who they are, not what they are. To prove that it isn't the clothes worn, that determine what one can do or not do.
Became part of the political revolution of the 1950/1960's as a statement against the staid gender roles established by society at the time. To more or less mock the rules governing the roles of gender in society.
More about 'identity' than sexuality, though it was often believed to be more about homosexuality than about changing the defined roles of gender.
History (Legend): The term is believed to have been coined in 1974 by Christopher Lonc.
To publicly dress in women's clothing, if a male, and to wear male clothing if a female, in obvious fashion. To over do it, go over the top, in order to make people notice the differences, notice that you are one gender, your clothing another's.
Noteworthy: A more political statement about identity, rather than sexuality, to 'fuck' with a person's mind or concept of roles of gender in society.