A uniform is a variety of costume worn by members of an organization while usually participating in that organization's activity. Modern uniforms are most often worn by armed forces and paramilitary organizations such as police, emergency services, security guards, in some workplaces and schools, and by inmates in prisons. In some countries, some other officials also wear uniforms in their duties; such is the case of the Commissioned Corps of the United States Public Health Service or the French prefects. For some organizations, such as police, it may be illegal for non-members to wear the uniform.

From the Latin unus (meaning one), and forma (meaning form).

Workers sometimes wear uniforms or corporate clothing of one nature or another. Workers required to wear a uniform may include retail workers, bank and post-office workers, public-security and health-care workers, blue-collar employees, personal trainers in health clubs, instructors in summer camps, lifeguards, janitors, public-transit employees, towing- and truck-drivers, airline employees and holiday operators, and bar, restaurant and hotel employees. The use of uniforms in commercial or public-service organizations often reflects an effort in branding and in developing a standard corporate image; it also has important effects^[which?] on the employees required to wear uniforms.

The term uniform may be misleading because employees are not always fully uniform in appearance and may not always wear attire provided by the organization, while still representing the organization in their attire. Academic work on organizational dress by Rafaeli & Pratt (1993) referred to uniformity (homogeneity) of dress as one dimension, and conspicuousness as a second. Employees all wearing black, for example, may appear conspicuous and thus represent the organization even though their attire is uniform only in the color of their clothing, not in its features. Pratt & Rafaeli, (1997) described struggles between employees and management about organizational dress as struggles about deeper meanings and identities that dress represents. And Pratt & Rafaeli (2001) described dress as one of the larger set of symbols and artifacts in organizations, which coalesce into a communication grammar.

Military uniform is the standardised costume worn by members of the armed forces and paramilitaries of various nations. Military dress and military styles have gone through great changes over the centuries from colourful and elaborate to utilitarian camouflage uniforms for field and battle purposes. Military uniforms in the form of standardised and distinctive dress, intended for identification and display, are typically a sign of organised military forces equipped by a central authority. Most military forces have developed several different uniform types.

Military personnel in most armed forces and some civilian officials may wear some or all of the following: combat uniform, service dress, dress uniform, full dress uniform, mess dress.

Uniforms can distinguish various categories of staff in medical institutions: doctors, surgeons, nurses, ancillary staff and volunteers.

Traditional female nurses' uniforms resemble uniforms (habits) worn by religious orders.