Crossdressing: the act of wearing clothing of the opposite sex. Transvestites are also included here, as the term is very similar. For the same matter, Drag queens and Drag kings should be considered here, but only if it's a major aspect of their character.
Note that crossdressing is only done by those who don't identify as the gender they present when dressing in the clothing of the opposite sex. Eddie Izzard crossdresses, while Calpernia Addams does not.