A fancy dress party (British English) or a costume party (American English) is a type of party, common mainly in contemporary Western culture, where guests dress up in costumes. Costumed Halloween parties are popular in the United States, Canada, Australia and New Zealand.
The origins of fancy dress parties in the United Kingdom can in some respects be traced to masked balls of the 18th century period. In the period to 1850, fancy dress balls were a typical part of the social life of music festivals.