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A cocktail dress or cocktail gown is a woman's dress worn at cocktail parties and semi-formal or "black tie" occasions.
Historically the length of a cocktail dress is long, often touching the ankle. When it is about 5 cm (2 inches) above the ankle it is called tea length and when it is almost touching the ankle it is called ballerina length, although ballerina length dresses usually fall into the evening gown category.
The modern definition of a cocktail dress is commonly taken to mean a shorter dress around knee length. It must be smart, such as the little black dress, and may have sequins or lace as a feature.
At semi-formal occasions, a less elaborate, shorter-length cocktail dress may be worn. Prior to the mid 20th century this type of dress was known as 'late afternoon'. When gentlemen are in business suits, ladies also wear dress suits or "good" afternoon skirt and dresses.
In the mid-20th century, some couturiers began describing full-skirted cocktail gowns as "dancing costumes". These are suitable for formal dances but not for balls, or other white tie affairs, where ball gowns are worn.
Christian Dior was the first to use the term "cocktail dress" to refer to early evening wear, in the late 1940s.
To read more about cocktail dresses, please click on the following link: http://en.wikipedia.org/wiki/Cocktail_dress