A character wearing clothing associated with their opposite gender.
Since this tag is somewhat culture-specific, it generally refers to one of two things. First, it applies to male/masculine characters wearing feminine clothing such as a dress, skirt, panties, high-heels, et cetera. Second, it applies to female/feminine characters wearing masculine outfits such as a traditional tuxedo and suit pants combination and also those wearing traditional pieces of clothing designated for males such as boxers, boxer briefs, et cetera.
Not to be confused with girly, which more often refers to the physical build and demeanor of the character (though their meanings do overlap).