A monorail is a railway system in which the track consists of a single rail or beam that serves as the sole support and guideway for passenger or freight vehicles. The term "monorail" comes from the words "mono" meaning "one" and "rail". Monorails are often elevated, but they can also run at grade, below grade, or in subway tunnels. They are different from trams and light rail systems because they are always separated from other traffic and pedestrians, and they do not use pantographs. Monorails are often found in airport transfers and medium-capacity metros.

Monorails can be driven by conventional electric traction motors or linear induction motors. They can be found in singular rigid format, articulated single units, or as multiple units coupled into trains. Monorail vehicles can be both manned and unmanned.

It is important to note that the term "monorail" is often used erroneously to describe any form of elevated rail or people mover. For example, the Detroit People Mover and the Vancouver Skytrain are often referred to as monorails, but they are actually elevated light rail systems that run on conventional steel dual rails on a massive wide beam.

There are several monorail systems in operation around the world, including the Tokyo Monorail, the Seattle Monorail, and the Walt Disney World Monorail System.