Informal wear, business formal or undress, also called business wear, corporate/office wear, tenue de ville or dress clothes, is a Western dress code for clothing defined by a business suit for men, and cocktail dress or trouser suit for women. On the scale of formality, it is considered less formal than evening gowns, dinner jackets and black lounge suits but more formal than casual wear. Informal or undress should not be confused with casual wear such as business casual or smart casual; most situations calling for "informal wear" will usually tolerate casual dress to varying extents.

The suit originated as leisure wear in the late 19th century but eventually replaced the frock coat as everyday wear in the city. After World War I, the suit was established as informal daily wear. Hats, such as trilbies or bowler hats, are sometimes worn with informal wear.

Informal wear is commonly worn for office use in professions such as politics, academia, law, finance and business, as well as to certain events such as job interviews in other sectors. It is a traditional dress code that aims to indicate respect to the situation and not draw attention.

The suit originated in Britain in the 19th-century as a leisurewear. Seeking a casual alternative to the knee-length, heavy frock coats then considered appropriate business dress, men began to wear lighter coats cut just below the seat when not engaged in business.

Standard suit-making fabric is fine combed wool, with the inclusion of cashmere in more expensive fabrics. Middle-price suits are often made of wool-polyester blends, whilst the cheapest are made entirely of polyester fabric.

This business suit (also known as the "sack suit" in North America, commonly by Brooks Brothers) became the standard business daywear for all men who were not engaged in physical labour. The vest was worn regularly with the suit up to the Second World War, but is rarely seen today, due to central heating in offices and the expense of construction. Until at least the early 1960s, it was common to wear a hat.

In general, business suits are characterized by three styles and a fourth fusion style. English suits are noted for having a "touch fit" to the wearer's body shape and carefully made padded shoulders. Italian suits are often slimmer, with higher armholes and highly shaped to complement a slim physique. Traditional American suits have lightly padded shoulders and loose natural fit with minimal shaping. Since the 1960s, designer brands (especially Polo Ralph Lauren) have created fusion style that brings a more shaped European look to the natural American cut.

Suits in Britain were often made in tweed, often with three pieces, and were worn outside the City of London. Tweed is made from uncombed wool, and, like all fabrics from the time, was thick and durable (18-ounce was considered medium-weight in the Edwardian era). A full tweed suit is less common today, with just tweed sports jackets more often worn, but is still used generally as everyday wear by some, and for outdoor sports such as shooting and angling. It is worn with appropriate accompanying clothes, much as any other suit; brown full brogues and wool ties are common items not worn with other types of suit.