Cosplay is the shortened form of Costume Play.It refers to the activity of dressing up in costume.

https://www.ccosplay.com/
The term "cosplay"originatesfrom Japan,where cosplay is a popular form of entertainment among children and young adults.Cosplay started in Japan,but it is gaining more and more popularity all around the waold.Many anime conventions hold contests for cosplay and for masquerade.Spider Man Costume
At the World Science Fiction Annual Conference held in Los Angeles, USA in 1984, Gao Qiao Shenzhi, a Japanese animator and chief executive officer of Studio Hard, a Japanese art studio, formally expressed a kind of self-acting, dressed-up performance art behavior as a harmonious English word "Cosplay", hence its name.