You can wear black at any hour of the day or night, at any age and for any occasion.
Christian Dior 1954
Wikipedia defines a little black dress as an evening or cocktail dress, cut simply, and often with a short skirt, originally made popular in the early 1920s by the well- known fashion icon, designer Coco Chanel. Intended by Chanel to be long-lasting, while versatile and affordable, as well as accessible to the widest market possible, and in a neutral color, its continued popularity is such that many refer to it by its abbreviation, 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, it can be 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 one's 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.
The photo, to the left is of Audrey Hepburn, as Holly Golightly, circa 1961. The Givenchy sleeveless sheath dress is his most copied creation.