evening gown
An evening gown is a long flowing women's dress usually worn to a formal clothes affair. It ranges from mid-calf to full-length, but never above the knees. Evening gowns are often made of a luxury fabric such as chiffon, velvet, satin, or silk.
If the dress is knee-length of above consider cocktail dress instead. Evening gowns are formal wear, so they tend to not be extremely revealing, in such cases cocktail dress may be a better tag.