Formal dress (UK) and formal wear (U.S.) are the general terms for clothing suitable for formal social events, such as a wedding, formal garden party or dinner, débutante cotillion, dance, or race. The Western style of formal evening dress, characterised by black and white garments, has spread through many countries; it is almost always the standard formal social dress in countries without a formal national costume.

A dress code is a set of rules governing a certain combination of clothing; some examples are black tie and morning dress. Formal dress is the grouping of all the dress codes which govern clothes worn to formal events. The traditional rules that govern men's formal dress are strictly observed; from these derive the evening dress variants worn on many occasions, such as high school prom dances, formal dances, and entertainment industry award programs.

The dress codes considered formal in the evening are white tie and black tie. In the UK, morning dress is standard formal day time clothing (a lounge suit being still considered informal dress), but in the U.S. morning dress is rare, having been replaced with the stroller and then the lounge, or business, suit. Morning dress, however, does remain de rigueur in certain settings in the Europe, Australasia, and Japan. Some countries still have a semi-formal daywear code, the stroller.

Terminology: formal

These terms are used by traditional etiquette and dress consultants (especially for weddings), while contemporary consultants use looser, modern definitions, in which white tie is styled as very formal or ultra formal; black tie as formal; and the traditionally informal lounge suit as pseudo-formal. Moreover, modern advisors recommend black tie for events traditionally considered to require formal dress, and alternatives for what would have been semi-formal events.

However, formal and semi-formal are unambiguous when it is known they are being used in a traditional setting, even though changing fashions can make these terms ambiguous; white tie and black tie on the other hand refer solely to the combination of relevant clothes themselves, regardless of their setting, and so are much less susceptible to misinterpretation.

Particularly in America, but also around the Western world, there has also been a relaxation regarding the dress codes themselves, since full formal dress (white tie or morning dress) is almost unheard of in many places. An example of such a variant is removing the traditional, classic black bow tie required by the black tie dress code in favour of a black or coloured regular tie and vest, a development which would have been unthinkable a few decades ago.