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Wiktionary
evening gown

n. another term for an '''evening dress''', an elegant dress worn by women.

WordNet
evening gown

n. a formal gown for evening wear [syn: dinner dress, dinner gown, tea gown]

Wikipedia
Evening gown

An evening gown is a long flowing women's dress or gown usually worn to a formal affair. It ranges from tea and ballerina to full-length. Evening gowns are usually made of luxurious fabrics such as chiffon, velvet, satin, organza, etc. Silk is a popular fibre for many evening gowns. Although the terms are used interchangeably, ball gowns and evening gowns differ in that a ball gown will always have a full skirt and a fitted bodice; in contrast, an evening gown can be any silhouette— sheath, mermaid, A-line or trumpet shaped—and may have an empire or dropped waist.

It corresponds to both men's semi-formal wear for black tie events and men's formal wear for white tie events.