cocktail dress
US /ˈkɑːk.teɪl ˌdres/
UK /ˈkɑːk.teɪl ˌdres/

1.
a woman's short, elegant dress worn on formal occasions
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She wore a stunning red cocktail dress to the party.
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A black cocktail dress is a versatile wardrobe staple.
a woman's short, elegant dress worn on formal occasions