cocktail dress 
noun
A smart dress suitable for formal social occasions.The event is black tie or leisure suit, and cocktail dress or formal gown for the ladies....
- Dressed in a ravishing cocktail dress of a silky dark blue, she smiled at both of them widely brown eyes twinkling.
- If it was an A-line cocktail dress with a dusting of floral accents, it would have been a much better fit within the overall collection.