Light rail or light rail transit (LRT) is a form of urban rail public transportation that generally has a lower capacity and lower speed than heavy rail and metro systems, but higher capacity and higher speed than street-running tram systems. The term is typically used to refer to rail systems with rapid transit-style features that usually use electric rail cars operating mostly in private rights-of-way separated from other traffic but sometimes, if necessary, mixed with other traffic in city streets. Modern light rail technology is highly flexible in how it can be used, and whether any given system is considered a true rapid transit system or not depends on its implementation.

The term light rail was devised in 1972 by the U.S. Urban Mass Transportation Administration (UMTA) to describe new streetcar transformations which were taking place in Europe and the United States. In Germany the term Stadtbahn (to be distinguished from S-Bahn which stands for Stadtschnellbahn) was used to describe the concept, and many in the UMTA (now called the Federal Transportation Administration) wanted to adopt the direct translation, which is city rail. However, the UMTA finally adopted the term light rail instead.

Light in this context is used in the sense of "intended for light loads and fast movement", rather than referring to physical weight, since the vehicles often weigh more than those on so-called heavy rail systems. The investment in infrastructure is also usually lighter than would be found for a heavy rail system. The American Public Transportation Association (APTA) in its Glossary of Transit Terminology defines light rail as: "An electric railway with a 'light volume' traffic capacity compared to heavy rail.

Light rail may use shared or exclusive rights-of-way, high or low platform loading and multi-car trains or single cars." However, some diesel powered transit calls itself light rail, such as the O-Train in Ottawa, Canada and River Line in New Jersey, United States, which use diesel multiple unit cars. In traditional transit terminology, these would perhaps be considered commuter rail lines, or branch lines, or interurbans. (If those lines had been electric, they would clearly be interurbans.)

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