A costume is something that is commonly worn by people who are pretending to be someone else or for other reasons. They are popular at social events, such as Halloween parties. Most superheroes wear costumes, and quite a few supervillains do as well.
At the Justice League Hall of Fame, a large number of wax sculptures of the JLA were on display, complete with their costumes. when a heatwave occurred during the Cosmic Crisis, the costumes were in danger of getting ruined, so Marvin, Wendy and Wonder Dog worked quickly to get them off.