Bus rapid transit (BRT) is a term applied to a variety of public transportation systems using buses to provide faster, more efficient service than an ordinary bus line. Often this is achieved by making improvements to existing infrastructure, vehicles and scheduling. The goal of these systems is to approach the service quality of rail transit while still enjoying the cost savings and flexibility of bus transit. The expression BRT is mainly used in the Americas; in Europe and Australia, it is often called a busway, while elsewhere, it may be called a quality bus.

Bus rapid transit takes part of its name from rail rapid transit, which describes a high-capacity urban public-transit system with its own right-of-way, multiple-car trains at short headways, and longer stop spacing than traditional streetcars and buses. BRT, however, uses buses on a wide variety of rights-of-way, including mixed traffic, dedicated lanes on surface streets, and busways completely separated from traffic.

Typical transit speeds of BRT systems range from 17 to 30 miles per hour (27 to 48 km/h), which can compare with surface running light rail.

A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on elevated rails above street level. Outside urban centers, rapid transit lines may run on grade separated ground level tracks.

Service on rapid transit systems is provided on designated lines between stations using electric multiple units on rail tracks, although some systems use guided rubber tyres, magnetic levitation, or monorail. They are typically integrated with other public transport and often operated by the same public transport authorities. Rapid transit is faster and has a higher capacity than trams or light rail (but does not exclude a fully grade separated LRT), but is not as fast or as far-reaching as commuter rail. It is unchallenged in its ability to transport large amounts of people quickly over short distances with little land use. Variations of rapid transit include people movers, small-scale light metro and the commuter rail hybrid S-Bahn.