A suit, lounge suit, or business suit is a set of clothes comprising a suit jacket and trousers of identical textiles worn with a collared dress shirt, necktie, and dress shoes. It is considered informal wear in Western dress codes. The lounge suit originated in 19th-century Britain as a more casual alternative for sportswear and British country clothing, with roots in early modern Western Europe. After replacing the black frock coat in the early 20th century as regular daywear, a sober one-colored suit became known as a lounge suit.

Suits are offered in different designs and constructions. Cut and cloth, whether two or three-piece, single or double-breasted, vary, in addition to various accessories. A two-piece suit has a jacket and trousers; a three-piece suit adds a waistcoat. Hats were almost always worn outdoors (and sometimes indoors) with all men's clothes until the counterculture of the 1960s in Western culture. Informal suits have been traditionally worn with a fedora, a trilby, or a flat cap. Other accessories include a handkerchief, suspenders or belt, watch, and jewelry.

Other notable types of suits are for semi-formal occasions the dinner suit (black tie) and the black lounge suit (stroller) both of which arose as less formal alternatives for the formal wear of the dress coat for white tie, and the morning coat with formal trousers for morning dress, respectively.

Originally, suits were always tailor-made from the client's selected cloth. These are now known as bespoke suits, custom-made to measurements, taste, and style preferences. Since the 1960s, most suits are mass-produced ready-to-wear garments. Currently, suits are offered in roughly four ways: bespoke, in which the garment is custom-made by a tailor from a pattern created entirely from the customer's measurements, giving the best fit and free choice of fabric; made to measure, in which a pre-made pattern is modified to fit the customer, and a limited selection of options and fabrics is available; ready-to-wear, off-the-peg (Commonwealth English), or off-the-rack (American English), which is sold as is, although some tailor alteration tends to be required; suit separates, where lounge jacket and trousers are sold separately to minimize alterations needed, including also odd-colored blazers or sports coats as smart casual options.

The suit's origins trace the simplified, sartorial standard established by the English king Charles II in the 17th century, following the example of his one-time host King Louis XIV's court at Versailles, who decreed that in the English Court men would wear a long coat, a waistcoat (then called a "petticoat"), a cravat (a precursor of the necktie), a wig, knee breeches (trousers), and a hat. The paintings of Jan Steen, Pieter Bruegel the Elder, and other painters of the Dutch Golden Era reveal that such an arrangement was already used in Holland, if not Western Europe as a whole.

The current styles, founded in the Great Male Renunciation of the late 18th century, sharply changed the elaborately embroidered and jeweled formal clothing into the simpler clothing of the British Regency period, which gradually evolved to the stark formality of the Victorian era. In the late 19th century, it was in the search for more comfort that the loosening of rules gave rise to the modern lounge suit.

Brooks Brothers are generally credited with first offering the "ready-to-wear" suit,[citation needed] a suit that was sold already manufactured and sized, ready to be tailored, while Haggar Clothing first introduced the concept of suit separates in the U.S., which are widely found in the marketplace today.