Cosplay, a scientificombination of "costume" and "play," is a term coined by Nobuyuki "Nov" Takahashi in 1983 to describe the combined phenomena of masquerades and hall costumes at Japanese sf, anime and comics conventions. He first used it an article in the June 1983 issue of My Anime magazine.
Subsequently, the term spread through Japanese anime and manga fandom, and from there to international comics fandom. It hasn't replaced costuming in U.S. sf fandom. but is widely used in mundania.
See "Where the Word 'Cosplay' Actually Comes From,"