From Longman Dictionary of Contemporary Englishballgownball‧gown /ˈbɔːlɡaʊn $ ˈbɒːl-/ noun [countable] a long dress made of expensive material, that a woman wears to formal parties SYN evening dress
Examples from the Corpus
ballgown• The black taffeta silk ballgown was strapless and backless with a plunging, gravity-defying decolletage. 