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In the European tradition, casual is the dress code that emphasizes comfort and personal expression over presentation, formality and conformity. More simply, "casual" can be defined as something relaxed, occasional, not planned, or informal. It includes a very wide variety of costume, so it is perhaps better defined by what it is not than what it is. The following are not considered casual wear:
- Ceremonial dress such as royal robes and full dress military costume
- Formal wear such as white tie.
- Semi-formal such as black tie.
- Informal business professional wear such as suits and ties. 
Although it can be considered "informal" in the senses of "not formal" or "suited for everyday use," informal attire actually refers to a dress code much more formal than casual wear, a step below Semi-formal wear. Casual wear should be done tastefully. No holes, tears, or stains. 
Jeans and a T-shirt have been described as the "casual uniform". With the popularity of spectator sports in the late 20th century, a good deal of athletic gear has influenced casual wear. Clothing worn for manual labor also falls into casual wear.
While utilitarian costume comes to mind first for casual dress, however, there is also a wide range of flamboyance and theatricality. Punk costume is a striking example. Madonna introduced a great deal of lace, jewelry, and cosmetics into casual wear during the 1980s. More recently, hip hop fashion has played up elaborate jewelry and luxurious materials worn in conjunction with athletic gear and the clothing of manual labor.
Casual wear is typically the dress code in which new forms of gender expression are attempted before being accepted into semi-casual or semi-formal situations. An obvious example is masculine jewelry, which was once considered shocking or titillating even in casual circles, and is now hardly noteworthy in semi-formal situations. Amelia Bloomer introduced trousers (of a sort) for women as a casual alternative to formal hoops and skirts. In a recent mirror image, sarongs and other skirts have been embraced by a few men of the European tradition as a casual alternative to formal trousers. Both of these innovations caused great embarrassment in formal circles.
The trend toward female exposure in the 20th century has also pushed the necklines of formal ball gowns ever lower and the skirts of cocktail dresses ever higher. For men, the exposure of shoulders, thighs, and backs is still limited to casual wear.
- "Attire Guide: Dress Codes from Casual to White Tie - The Emily Post Institute". The Emily Post Institute, Inc. Retrieved 2017-05-05.
- Allan, Patrick. "What All of Those Confusing Dress Code Terms Really Mean". Lifehacker. Retrieved 2017-05-05.