Rapid transit, mass rapid transit (MRT) or rail rapid transit (RRT) and commonly referred to as metro, is a type of high-capacity public transport that is generally built in urban areas. A grade separated rapid transit line below ground surface through a tunnel can be regionally called a subway, tube, metro or underground. They are sometimes grade-separated on elevated railways, in which case some are referred to as el trains – short for "elevated" – or skytrains. A common alternative term for rapid transit in North America is heavy rail. Rapid transit systems are usually electric railways that, unlike buses or trams, operate on an exclusive right-of-way, which cannot be accessed by pedestrians or other vehicles.

Modern services on rapid transit systems are provided on designated lines between stations typically using electric multiple units on railway tracks. Some systems use guided rubber tires, magnetic levitation (maglev), or monorail. The stations typically have high platforms, without steps inside the trains, requiring custom-made trains in order to minimize gaps between train and platform. They are typically integrated with other public transport and often operated by the same public transport authorities. Some rapid transit systems have at-grade intersections between a rapid transit line and a road or between two rapid transit lines.

The world's first rapid transit system was the partially underground Metropolitan Railway which opened in 1863 using steam locomotives, and now forms part of the London Underground. In 1868, New York opened the elevated West Side and Yonkers Patent Railway, initially a cable-hauled line using stationary steam engines.

The term Metro is the most commonly used term for underground rapid transit systems used by non-native English speakers. Rapid transit systems may be named after the medium by which passengers travel in busy central business districts; the use of tunnels inspires names such as subway, underground, Untergrundbahn (U-Bahn) in German, or the Tunnelbana (T-bana) in Swedish. The use of viaducts inspires names such as elevated (L or el), skytrain, overhead, overground or Hochbahn in German. One of these terms may apply to an entire system, even if a large part of the network, for example, in outer suburbs, runs at ground level.

In most of Britain, a subway is a pedestrian underpass. The terms Underground and Tube are used for the London Underground. The North East England Tyne and Wear Metro, mostly overground, is known as the Metro. In Scotland, the Glasgow Subway underground rapid transit system is known as the Subway. In Ireland, the Dublin Area Rapid Transit is despite the name considered a commuter rail due to usage of mainline railways.

In France, large cities, such as Paris, Marseille and Lyon, use the term métro. Also the smaller cities of Lille and Rennes have a light metro. Furthermore, Brussels in Belgium, and Amsterdam and Rotterdam in the Netherlands also use métro or metro for their systems.

Several Southern European countries also use the term metro (Iberian Peninsula) or metropolitana (Italy) for rapid transit. In Spain, such systems are present in Madrid, Barcelona, Bilbao and Valencia. In Portugal, Lisbon has a metro. The Italian cities of Catania, Genoa, Milan, Naples, Rome, Brescia and Turin also have rapid transit systems.

In Germany and Austria they rapid transit is known as U-Bahn, which are often supported by S-Bahn systems. In Germany, U-Bahn systems exist in Berlin, Hamburg, Munich, Nuremberg and Fürth, while in Austria such a system exists in Vienna. In addition, the small, car-free town of Serfaus in the Austrian state of Tyrol also features a short U-Bahn line. There are no U-Bahn systems in the German-speaking part of Switzerland, but the city of Lausanne has its own, small métro system. In Zurich, Switzerland's largest city, a project for a U-Bahn network was stopped by a referendum in the 1970s and instead its S-Bahn system was developed further. Other Central European countries also have metro lines, for example in the cities of Budapest (Hungary), where it is called metró, Prague (Czech Republic) and Warsaw (Poland) – the latter two systems also use the term metro.