Definition of fancy dress in US English:
 fancy dress
nounˈˌfansē ˈdresˈˌfænsi ˈdrɛs
An unusual or amusing costume worn to make someone look like a famous person, fictional character, or an animal, especially as part of a theme at a party.
 schoolchildren were in the procession in fancy dress
as modifier a fancy dress competition
 Example sentencesExamples
-  Don't bother hiring an expensive fancy-dress costume.
 -  The costume party is formatted as a multi-venue themed fancy-dress art party and is attended by 8000 thousand partygoers - who arrive in teams in matching costumes - somehow related to the theme.
 -  They talk of going to future fancy-dress parties and of the costumes worn there.
 -  Following this is a fancy-dress competition, in which everyone in a Von Trapp costume will be invited on stage.
 -  Then of course there is the famous fancy-dress competition in which everyone who has arrived in costume is invited onto the stage to show off.
 
 Synonyms
disguise, veil, false face, domino, stocking mask