noun: 1. The act or practice of dressing up as a character from a work of fiction, such as a comic book, video game, film, etc. 2. The act of, or an instance of, pretending to be someone in a deceptive manner. verb tr.: 1. To dress up as a fictional character in cosplay. 2. To pretend to be someone in a deceptive manner. verb intr.: To take part in cosplay.