Crossplay (the term is a portmanteau of crossdressing and cosplay) is a type of cosplay in which the person dresses up as a character of a different gender. Crossplay's origins lie in the anime convention circuit, though, like cosplay, it has not remained exclusive to the genre.
In most countries that play host to hobbyists who would call themselves cosplayers, female-to-male crossplayers (females costumed as male characters, sometimes abbreviated "FtM") are far more common, due to a variety of social and cultural factors.
Many (or even most) females will crossplay for the same reasons that they would cosplay – because they like the character and/or the costume, and wish to represent that. In Japan, female costumers tend to dominate (in numbers) the field of cosplay in general, often portraying a huge assortment of colorful characters regardless of gender.
As bishōnen are portrayed in manga and anime as liminal beings, it is considered "easier" for a female to cross-play as a bishōnen than it would be for her to crossplay as a male character from a Western series.
In some countries, FtM crossplayers do not catch the public eye as any stranger than their non-crossplay counterparts. However, in yet other countries, there is a social stigma attached to FtM crossplay: some, particularly those outside the cosplay community, suspect FtM crossplayers as having penis envy, being transgender, or having a lesbian sexual orientation. On the other hand, within the cosplay community, there is very little stigma attached to FtM.
In contrast to some of these social stigmas, most females who crossplay are not actually homosexual or transgender and simply enjoy the art of dressing up as their favorite character. For these crossplayers, it is no different from an actress playing a male role.
Male-to-female crossplayers, (males costumed as female characters, sometimes abbreviated "MtF"), are somewhat more common outside Japan. Originally, in America, a popular anime series for MtF crossplayers was Sailor Moon, creating "humorous effect and social levity". Due largely to the nature of most costumes from the series (skintight tops and impossibly short skirts), this led to a negative stereotype which still exists both inside and outside of the anime community: that of the overweight, hairy-legged Sailor Soldier. This extremely derogatory stereotype has been frequently parodied, sometimes through intentionally ironic genderplay crossplay. "Sailor Bubba" (a staple of Anime Central, further enshrined as a bobble-head doll) and Man-Faye are primary examples of this.