Cosplay: A type of performance art in which participants outfit themselves as specific characters, often based on science fiction, fantasy fiction, Harry Potter novels, or Japanese anime. The object is not merely to "dress up" but to become one's character. Dressing as a character of the opposite sex is called crossplaying.
Cosplay, a portmanteau of costume and play, was coined in 1984 by Nobuyuki (Nov) Takahashi, whose company, Studio Hard, publishes several anime magazines in Japan. The Japanese word--kosupure--is based on the English term. People with an excessive interest in cosplay are known by a Japanese term, otaku, that has an older meaning as a second-person honorific.
There is big money to be made in cosplay, and not only from the costumes themselves. One Japanese cosplay-costume company has diversified into cosplay changing rooms, according to Wired magazine, which features a monthly Japanese Schoolgirl Watch:
Many Japanese gals lead double lives: Mild-mannered students in plain-Jane uniforms by day; French maids, furries, and goth Lolitas by night. Legions run around Tokyo, wheeling suitcases full of makeup and costumes. But Superman had a phone booth — where are schoolgirls supposed to suit up? Luckily, Japanese company COS-Pa has introduced tiny dressing rooms for women in the trendy Shibuya district, where 500 to 700 yen (roughly $5 to $6) buys 30 minutes of private mirror time, free Wi-Fi, beauty supplies, and a nonalcoholic beverage. Says COS-Pa's owner firmly: "Ladies should not have to get beautiful in a bathroom."
According to its Wikipedia page, COS-Pa (or Cospa) is an acronym for Contents Communication Service Partner.