Cocktail dress...

A cocktail dress or cocktail gown is a woman's dress worn at cocktail parties, and semi-formal occasions. The length of cocktail dresses varies depending on fashion and local custom. The length ranges from just above the knee to 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.

At semi-formal occasions, less elaborate, shorter-length cocktail dresses 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 dress.

By the 1980s a dress was considered to be cocktail less by where the hem fell, but more by the level of sumptuousness the garment attained. 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.

Cocktail Dress: Little Black Dress

A little black dress is an evening or cocktail dress, cut simply and often quite short. Fashion historians ascribe the origins of the little black dress to the 1920s designs of Coco Chanel, intended to be long-lasting, versatile, affordable, accessible to the widest market possible and in a neutral color. Its ubiquity is such that it is often simply referred to as the "LBD."
The "little black dress" is considered essential to a complete wardrobe by many women and fashion observers, who believe it a "rule of fashion" that every woman should own a simple, elegant black dress that can be dressed up or down depending on the occasion: for example, worn with a jacket and pumps for daytime business wear or with more ornate jewelry and accessories for evening. Because it is meant to be a staple of the wardrobe for a number of years, the style of the little black dress ideally should be as simple as possible: a short black dress that is too clearly part of a trend would not qualify because it would soon appear dated.