or kosupure as it is written in katakana
, is the shortened term for costume play, in which professionals or amateurs will dress up in costumes portraying some sort of fictional character. Some costumes can be bought but most are custom created and involve a lot of work. Popular among anime fans, this phenomenon can be seen at nearly every major anime conference in the world.
Although it has been around in Japan since the sixties, when fans came together and celebrated their common interest as Fujiko and Lupin, it didn't catch on in North America until the late eighties, when Project A-kon, the Texas based anime convention, announced it as an official event. It has since grown in popularity in the west and is now considered a staple of the anime community. Despite this, cosplay is still generally frowned upon by the general public in the west, since it is still associated with weirdos, otakus, and sci-fi nerds.
In Japan it happens to be a bit more accepted (but only at appropriate events), and is not limited to just anime. Many television commercials feature models or actresses cosplaying as something to sell their product. The most popular and simple cosplay is the catgirl. All that's necessary to pull this one off is a pair of pointy cat ears, but there are also many other accessories such as a tail, (thanks to the influence of DiGi Charat) oversized bells, paw shaped boots, and big fuzzy paw mittens. Variations such as dog-girls and rabbit-girls (not the playboy kind) have also emerged. Other popular themes include the generic japanese schoolgirl (complete with sailor top and floppy socks), the magical girl, the french maid, the nurse, company product (see Bisuke-tan. Basically, you put a product or some other item on on your head that normally doesn't belong there to symbolize your affiliation with the related organization/company), or combinations of them all, like Magical Nurse Witch Komugi and Magical Maid Koyori. There is even pornographic cosplay, like the BLT image/story series that feature famous japanese porn stars cosplaying as popular anime or video game characters.
Both males and females cosplay, but like most fashion, the female costumes are almost always more elaborate and interesting. Male cosplays are somewhat limited to "throwing a big overcoat on and calling yourself some character from one of the final fantasy games." While there are some impressive male ones, such as the shitennou suits designed for the Sailor Moon Live Action television series, female clothing still has more options and lots of frill to play around with; so much so that crossplay is on the rise, where (usu.) males jokingly cosplay as females. However, like any hobby, there are some crossplays that start crossing the line between amusing and creepy. One type is known as Masking or Second Face, and it is generally not widely accepted anywhere, even among cosplayers. This is limited to anime, but is even considered by anime fans as overdoing it, and has overt sexual tones. It involves wearing a tight skin-colored body suit, wearing a costume over it, and then topping it off with an anime mask. Still, these are a minority, with the majority seeing cosplay as simply a fun thing to do, and constantly challenge themselves to come up with better ideas and outfits all the time.
Note: This isn't something I do, as I don't have the money, patience, or desire, but I started observing it (both amateur and pro) quite a bit after seeing my favorite person, Momoi Halko, pull off a perfect Komugi complete with rabbit ears and angel wings. A lot really is very interesting.